1983 15-footer Maximum Transom Weight

Optimizing the performance of Boston Whaler boats
Mr T
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1983 15-footer Maximum Transom Weight

Postby Mr T » Mon Mar 27, 2017 11:08 am

I am looking at a 1983 15-footer that has a 1983 Mercury 50-HP two-stroke-power-cycle engine. I'd like to update it to a newish four-stroke-power-cycle engine in the 50 to 75-HP range. I don't have any idea what is the maximum weight I can hang back there.

Does anyone have first hand info on what is possible here?
Last edited by Mr T on Fri Jun 02, 2017 1:36 am, edited 1 time in total.

jimh
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Re: 1983 15-footer Maximum Transom Weight

Postby jimh » Mon Mar 27, 2017 1:47 pm

I used to have a classic 1976 SPORT 15 and it also had a 1976 Mercury 50-HP.

I don't believe that Boston Whaler ever published a maximum transom weight limit for the c.1983 classic 15-foot SPORT or similar open boat. In 2004 I initiated an informed discussion on the topic of maximum transom weight for hulls like this one which had never been assigned such a specification. At that time I wrote:

Generally the classic Boston Whaler hulls were not rated by the factory for a maximum engine weight limit. Recently, newer recreational hulls designs have been given a specification for maximum engine weight. Certain hull designs from the classic era are still being produced by Brunswick Commercial and Government Products (CGP), and these hulls now have been given a maximum engine weight rating. Using the current CGP hulls and their ratings as a guideline, it may be possible to develop a sense of the maximum engine weight rating for the older recreational hulls.

From the 2004 CGP Catalogue, I would suggest the following correspondences:

CGP Model / Maximum Engine Wt. / Similar classic hull
15 ALERT = 264-lbs --> SPORT 15
17 ALERT = 330-lbs --> MONTAUK 17


In the discussion that followed, a proposal was made that the calculation of maximum transom weight for the CGP hulls must be taken as a proportion of their hull weight, and that to assign the same maximum transom weight to the lighter recreational hulls would be inappropriate. A fellow (Moe) proposed that the recreational hulls be down-rated in their maximum transom weight so that the maximum transom weight would be the same percentage of the hull weight as was used for the commercial hulls. His calculations came up with these values:

Model / Hull Wt. / Maximum Engine Wt.
SPORT 15 = 580-lbs --> 249-lbs
STANDARD 17 = 850-lbs --> 281-lbs
MONTAUK 17 = 950-lbs --> 285-lbs


Reflecting on those specifications and the calculated weights derived from them, I would note that there have been many, many MONTAUK 17 hulls re-powered with an E-TEC 90-HP engine that weighs 320-lbs and there has never been even a suggestion or hint or complaint that there was too much transom weight. Note that the 320-lbs E-TEC is less than the maximum (330-lbs) transom weight recommended by Boston Whaler for the CGP hull, and that was my initial sense of what ought to be a reasonable guide for the recreational hulls. If you can find that assumption to be reasonable--and I do--then you could also assume that for a SPORT 15 the maximum engine weight should be limited to about 264-lbs.

jimh
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Re: 1983 15-footer Maximum Transom Weight

Postby jimh » Mon Mar 27, 2017 1:58 pm

As for what sort of 50-HP to 70-HP outboard engine using four-stroke-power-cycle design can be found that weighs about 264-lbs, I leave that for you and others to research. I do note that Brunswick's Mercury brand has a 60-HP FOURSTROKE model that weighs 260-lbs. And Yamaha has an F60 that weighs 249-lbs. I am sure there are other choices available.

flymo
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Re: 1983 15-footer Maximum Transom Weight

Postby flymo » Mon Mar 27, 2017 4:30 pm

Many 15-footers were originally rigged with OMC 60 or 70-HP outboard engines which weighed about 270-lbs. On my 15-footer, I have a Yamaha F70 which weighs a little bit less than [270-lbs] and requires no separate oil tank like the old OMC engines did. You should be fine going in that range [270-lbs]. I would not recommend putting more [than 270-lbs] weight on there, nor do I know of a modern 75 that is that light.

More generally, you should not restrict your choices to four-stroke engines. Modern, direct injection two-strokes are every bit as quiet and easy to live with.

jimh
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Re: 1983 15-footer Maximum Transom Weight

Postby jimh » Tue Mar 28, 2017 10:35 am

By the way, if that older Mercury 50-HP engine is the four-cylinder 44-cubic-inch displacement engine, with electric starting and perhaps even with power tilt and trim, you should give some thought to just keeping that engine on the Boston Whaler SPORT 15. That old Mercury is a great outboard engine, and if you keep the soft components like the fuel hoses and the electrical wire insulation in good condition, that engine will run forever--assuming you don't forget to pre-mix the fuel and oil. When I owned that engine I once asked my Mercury dealer about upgrading to something newer. He looked at me like I was crazy and told me that old 50-HP was one of the best outboard engines ever made.

Considering the price of gasoline fuel in the USA these days--about $2 per gallon--it would make no sense to invest $5,000 in a new outboard engine on the notion of saving some fuel expense. A 15-footer is a great boat but you can only stand to sit in one for a couple of hours at the most, and that means your annual engine hours will be low. And when something mechanical gets to be over 30-years-old there is a bit of fun and pleasure in keeping it running.

Reliability is always a paramount concern. I recall an anecdotal report about the outboard engines used in Antarctica: they used old, low-tech, simple, carburetor two-stroke engine that were easy to keep running and could be repaired on location. They also liked them because you could pull-start them. Engines that can't be started except with a strong battery can really degrade reliability in cold climates.

Mr T
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Re: 1983 15-footer Maximum Transom Weight

Postby Mr T » Tue Mar 28, 2017 12:25 pm

Thank you to all for the feedback- it appears a number of options are in reach based on the information.

Jim, from what I gather from the owner, it is a 4 cylinder with power tilt and trim. He is the second owner of the hull and motor since 1985. He states there are few hours on the rig, although that can't be confirmed.

Gas prices in CA are steady at $3 per gallon, regardless, it is a big decision to spend that kind of money on a newer motor. Your comment on the reliability of the motor is heartening. My major concern is the fear of a breakdown somewhere on the water. From what you say it appears that I may be in good shape here. A secondary concern is range, I will be keeping 6-10 gallons of gas and the fuel comsumption "should be" somewhere around 5 gallons/hour for an older 50 hp such as this Mercury motor.

I'd love to keep it all original if possible it looks great as it is sits
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Whal
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Re: 1983 15-footer Maximum Transom Weight

Postby Whal » Tue Mar 28, 2017 5:32 pm

I have a 1999 Alert 17 and I have a Yamaha F70 and a Tohatsu 6 hp kicker. The total weight is 319 lbs and it is just right, I agree with jimh and Boston Whaler with the 330 pound limit for that hull and I presume that the weight limit for the 15 footer is also right. Many will tell you that you can put a heavier motor on it but I will tell you from personal experience that it will make your boat ride rougher and it will handle time a tank, not to mention taking an occasional wave over the transom. These Whaler hulls handle like a sports car with the proper amount of weight on the transom and they handle and ride like a tank with too much weight on the transom. Your 15 would be fantastic with a Yamaha F70 that weighs 257 lbs. I think Boston Whaler engineers know how much weight is right for their hulls.

jimh
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Re: 1983 15-footer Maximum Transom Weight

Postby jimh » Wed Mar 29, 2017 7:35 am

An older 50-HP might consume 5-GPH at maximum throttle, but you won't be running at maximum throttle all the time.

The engine in your pictures looks like the classic four-cylinder 44-cubic-inch engine I had. The only problem I had with that engine was the carburetor float valves would stick. I had the carburetors disassembled and cleaned, and it ran very well after that. I sold it many years ago, but the I am still in contact with the buyer. That engine continues to run very well for him. It was a a 1976, so even older than the 1983 you might buy. Here is my old boat

Image

Mr T
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Re: 1983 15-footer Maximum Transom Weight

Postby Mr T » Wed Mar 29, 2017 7:17 pm

I bought the rig. There will be some work needed on my end to replace some wiring under the cowling. As I had feared, after many years, the wires simply cracked and the insulation breaks off of it, exposing bare wires that are badly corroded. I will be investigating a source to buy a new harness.

The motor has power tilt and trim, and, while everything is there and functions fine, it is very unsightly and takes up a good deal of room in the stern.

The carburetors are leaking a bit, but I feel a simple rebuild is going to remedy that. I was not able to get compression testing done. With that said, the motor fired up easily and with only a short turn of the key, and idles very smoothly. Mild acceleration in neutral to under 2,000-RPM showed no signs of skipping or stumbling. Owner told me in the last twenty years he has run it perhaps 10 to 20 times per season, so I don't expect many hours are on the motor. Aside from the lower end where the water pump service is done, there are no signs of the head bolts or any connectors aside from those fuel related that have been disturbed.

Based on this and input from a number of people, I have concluded I am going to try to keep the motor running as long as I can.

The hull is very solid. A tap test, while not the best arbiter of moisture, nevertheless only showed a few spots where water has intruded, as expected around open screw holes.

I will be picking the rig up over Memorial Day and hope to have some time to test on the water. After meeting the owner and spending some time talking, I'm comfortable that he is honest in his experience with the boat and that the rig will perform well for me.

I'm hoping to see if anyone here knows a good source of parts, I've used a site called Maxrules in the past and will try that again, but I am not sure they are still around.

Mr T
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Re: 1983 15-footer Maximum Transom Weight

Postby Mr T » Fri Mar 31, 2017 10:06 am

I have a 2004 Yamaha 90HP two-stroke engine that weighs 266-lbs. I am tempted to put that on, but the idea of overpowering the hull is not exciting.

conch
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Re: 1983 15-footer Maximum Transom Weight

Postby conch » Sat Apr 01, 2017 10:06 am

I would hang that 90hp Yamaha on in a heartbeat,or you can sell that 35 year old Mercury and the 90 Yamaha and put the very desirable F70 Yamaha four stroke on.

2004 Yamaha 90hp usually sell for about $4,500 in good condition in south Florida.
Chuck

conch
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Re: 1983 15-footer Maximum Transom Weight

Postby conch » Sat Apr 01, 2017 10:30 am

MrT--as you indicated in your posts above, "the idea of overpowering the hull is not exciting." Then the chance of a fire and explosion from running that old engine certainly should be exciting enough.

"The wires simply cracked and the insulation breaks off of it, exposing bare wires that are badly corroded...the carburetors are leaking a bit"

Mr T
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Re: 1983 15-footer Maximum Transom Weight

Postby Mr T » Sat Apr 01, 2017 4:36 pm

Conch--you are correct: I'm not excited about that one bit. Both problems are relatively minor as long as I can get parts--and I have a number of inquiries out already for that. I have no intention of running that engine in its current state, both problems will be repaired and safe before the engine is used.

Mr T
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Re: 1983 15-footer Maximum Transom Weight

Postby Mr T » Sun Apr 09, 2017 12:45 am

I have made the decision to install the Yamaha 90 hp 2 stroke motor on the SPORT 15 hull.

There are a number of videos online that show this combination of motor and hull and the trim looks level with that setup. I believe that prudent use of the throttle will preclude any potential problems with the horsepower.

jimh
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Re: 1983 15-footer Maximum Transom Weight

Postby jimh » Sun Apr 09, 2017 2:08 pm

Egads--a SPORT 15 with 90-HP is a rocket ship. Be careful.

Mr T
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Re: 1983 15-footer Maximum Transom Weight

Postby Mr T » Sun Apr 09, 2017 2:23 pm

jimh wrote:Egads--a SPORT 15 with 90-HP is a rocket ship. Be careful.



I am and will continue to be a prudent and rather tame operator of boats, I think that given the fact that the motor is a good weight match, is newer and has less operating hours and that I already possess it are strong arguments in favor of mounting it on the hull.

I'll play nice and easy with it!

porthole
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Re: 1983 15-footer Maximum Transom Weight

Postby porthole » Mon Apr 10, 2017 10:07 am

I had a 1983 CENTER CONSOLE 15. It came with a 70-HP Mercury that was supposedly re-worked to be an 85-HP engine. The boat was a rocket and a lot of fun, especially out on small craft days. The boat would easily do 45 knots. Near the end of the first season I had to be towed in. A hole in the bottom of the block that was missed on inspection. I had planned to re-power the boat when I bought it, but not right away.

I put on a Honda 50-HP engine. You can see by the pictures in my photobucket account that with all the stuff I did to the boat, the water line did not change. Key changes were:

--heavier engine
--moved battery to the console
--I had a 28 gallon fuel tank made by SP Sheetmetal in New Jersey (marine fuel tank fabricator)

The boat with the 50 HP Honda, although quieter and more fuel efficient, was significantly slower and underpowered. Top speed with one person and full fuel was barely 30 knots on flat water. That was after multiple propeller swaps.

"Porthole III" 1983 15' Boston Whaler center console
Last edited by porthole on Fri Apr 14, 2017 11:10 am, edited 3 times in total.
Thanks,
Duane
2016 World Cat 230DC
1999 Outrage 21, Yamaha SW Series II 200
1997 Outrage 18, Yamaha 125
1983 15 SS, Honda 50
1980 42 Post
1983 34 Luhrs 340 SF

Mr T
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Re: 1983 15-footer Maximum Transom Weight

Postby Mr T » Mon Apr 10, 2017 4:28 pm

Thanks for the information on your experience.

[Changed topic to electrical wiring. Please start a new thread in SMALL BOAT ELECTRICAL to discuss electrical wiring on small boats. Thanks--jimh]

porthole
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Re: 1983 15-footer Maximum Transom Weight

Postby porthole » Tue Apr 11, 2017 10:41 am

I did everything except fabricate the tank and console hand rail.
Thanks,
Duane
2016 World Cat 230DC
1999 Outrage 21, Yamaha SW Series II 200
1997 Outrage 18, Yamaha 125
1983 15 SS, Honda 50
1980 42 Post
1983 34 Luhrs 340 SF

Mr T
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Re: 1983 15-footer Maximum Transom Weight

Postby Mr T » Tue Apr 11, 2017 4:33 pm

[Changed TOPIC. Please use SMALL BOAT ELECTRICAL for your new topic.--jimh]

jimh
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Re: 1983 15-footer Maximum Transom Weight

Postby jimh » Wed Apr 12, 2017 9:05 am

PLEASE do not turn this topic into a discussion of electrical components. The topic of this thread is the maximum transom weight of a 1983 15-footer and its PERFORMANCE.

To discuss electrical components, please start a new thread in SMALL BOAT ELECTRICAL.

Mr T
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Re: 1983 15-footer Maximum Transom Weight

Postby Mr T » Sun May 28, 2017 12:37 am

I have come home today with the boat. The trip was 180-miles, and I noted that the Bearing Buddy seals were leaking from the back as a classic symptom of "adding a shot every time I run it."

After a four-hour drive with some apprehension, we got back. We stopped at a few shops to get pricing on an engine.

After discussing with my insurance company, the Yamaha 90-HP two-cycle motor is not an option; they will not insure the combination of this 15-foot hull with 90-HP on the transom.

I was distressed to hear the cheapest engine is a Mercury 60 FOURSTROKE starting at $6,300, a Honda BF50 starting at $6,800, and a Yamaha F50 starting at $7,100. All these prices were for a motor-only and no rigging, controls, or propeller. I did not obtain a quote for a Suzuki or Tohatsu engine at the time, but I did an internet check and found them in a similar price range.

Being very dismayed at these numbers, we are going to investigate replacing the corroded wiring on the Mercury 50-HP two-stroke-power-cycle engine for now.

Mr T
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Re: 1983 15-footer Maximum Transom Weight

Postby Mr T » Sun May 28, 2017 12:40 am

I also got a quote of $8,300 for an Evinrude E-TEC 50 hp outboard engine, and that was for the engine-only; no propeller, controls or rigging in that price. Wow.

jimh
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Re: 1983 15-footer Maximum Transom Weight

Postby jimh » Mon May 29, 2017 10:26 am

Your price survey of new outboard engines in the 50 to 60-HP range is interesting. That the least expensive engine is a Mercury does not surprise me. There is an old saying regarding product pricing: the measure of the value in a product is how easily its price can be raised.

The c.1983 Mercury 50-HP engine ought still to be useful, as long as the basics of the power head are in good condition. With some attention to the carburetors and some care for the electrical wiring, that engine should be able to run for a long time. The 1976 Mercury 500, the same engine as yours, that I had on my SPORT 15 when I sold it is still running and delivering very good performance.

These older carburetor two-stroke-power-cycle engines are going to use more fuel, but with just a 50-HP engine it won't be like you'll be burning more than an average of perhaps 2-GPH. What might you save in fuel with a new engine? Maybe 1-GPH. A gallon of fuel is about $2.50. When you divide $8,000 by 1-hour/$2.50-saved, you get 3,200-hours before you are going to earn back the cost of a new engine in saved fuel. There is really no rational justification to throw out a working 50-HP engine and replace it with a new one on the basis you will eventually save money on fuel. That just won't happen.

Of course, there are many irrational reasons to treat yourself to a new outboard engine, and there are many who indulge in that pleasure, including me.

Mr T
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Re: 1983 15-footer Maximum Transom Weight

Postby Mr T » Mon May 29, 2017 1:48 pm

Jim--my primary consideration is not fuel economy, it is essentially two factors.

1. The primary concern is reliability- the areas I will fish, (the California delta), is comprised of hundreds of miles of waterways, estuaries.sloughs, and river. There are a number of locations where cell phone coverage is non-existent, and vhf radio range is limited. In the case of a breakdown, it is quite possible I will be waiting for a long time for another boater to pass by.

2. A secondary consideration are the restrictions placed on 2 stroke motors in California. There are a number of waterways that prohibit two stroke motors compeletely, including 3 star rated ETEC models. [This topic has been separated from this discussion into a separate thread.--jimh]

With that said- I did place an order today for a new stator, trigger and wiring harness assemblies, new trim and tilt hose assemblies, and steering cable. The total cost is well under 700 dollars, a pleasant surprise frankly.


Given your positive comments regarding this model of motor, which I have buttressed with a number of my own inquiries, I have decided to stay with this engine, after updating all the suspect electrical connections.

The motor itself is in excellent shape and shows no sign of major work ever being performed.

If it fails, I will cross that road at that time. Until then I am going to keep the rig the way it came from day one- I like the look!

El Rollo
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Re: 1983 15-footer Maximum Transom Weight

Postby El Rollo » Tue Jun 06, 2017 1:58 pm

I have a 1988 Boston Whaler 15-footer with a Yamaha F70 four-stroke-power-cycle engine mounted on a Bob's Machine Shop 5-inch manual jackplate. This is a re-power from the original Yamaha 70-HP two-stroke-power-cycle engine. I'm am 100-percent satisfied with the way the boat sits and the outboard performs. I've had the F70 on the boat for over three years now and have not noticed a single stress crack around the transom post re-power.

The boat underwent a major customization, which included additional weight from the original factory layout.

I had originally wanted to re-power with an E-TEC 75 HP, but the dealer wouldn't install it citing it would be 5-HP over the boat's rated capacity. While I love the E-TEC on our 18 Outrage, and would have liked the E-TEC 75 on my 15, I am glad I didn't re-power with the Evinrude, as the additional weight would have been too much for my current configuration.

jimh
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Re: 1983 15-footer Maximum Transom Weight

Postby jimh » Thu Jun 08, 2017 4:11 pm

Dry weight of outboard from manufacturer specifications

Yamaha F70 = 257-lbs
Evinrude E-TEC 75 = 320-lbs

That is quite a difference in weight, 63-lbs, and particularly if you had to use a jack plante for set back, which probably adds at least 20-lbs of additional transom weight.

The E-TEC 75 with a Bob's Machine Shop 5-inch adjustable setback bracket would be a total of 340-lbs on the transom. Using the imputed maximum transom weight figure of 250-lbs as mentioned (far above in this discussion), the E-ETEC and jack plate combination would exceed the recommended transom weight by 90-lbs or a factor of 36-percent. That seems to be pushing the limit, particular for the boat under discussion, a 1983 model that is now 34-years-old. I don't believe in the notion that boats become stronger as they get older and have more use time; indeed, just the opposite is more reasonable.