1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Optimizing the performance of Boston Whaler boats
Kenb
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1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby Kenb » Tue Feb 04, 2020 8:05 pm

I bought a new Mercury 25-HP FOURSTROKE for my 1969 SUPER SPORT 13.

Here are propeller suggestion from a table from Mercury
    9"x12.5", 600-1000lbs, up to 16
    9.25"x11", 900-1300lbs, 13' to 16
    9.5"x9.5", 1100-1700lbs, 13' to 17
    9.75"x8", 1500lbs+, 16'+

I don't know which propeller [to buy]. I am guessing I should buy a 9.5 x 9.5-pitch [part number] 48-896892A40

The engine doesn't have power trim, but power trim can be added for $1,000 additional cost.

Q1: How much does adjusting trim help for varying loads?

If the engine [mounting] height is dialed in, and if the propeller is correctly matched to the engine:

Q2: how much does the tilt angle matter?

jimh
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Re: SUPER SPORT 13 with Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby jimh » Tue Feb 04, 2020 10:55 pm

There is no way to predict the performance and the optimum propeller without additional information.

Q3: What is the gear ratio of the engine?

Q4: What is the maximum recommended engine speed?

Q5: What is the recommended full-throttle engine speed range?

jimh
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Re: 1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby jimh » Tue Feb 04, 2020 11:01 pm

Kenb wrote:Q1: How much does adjusting trim help for varying loads?


On a 13-footer I would expect having power trim would be a luxury. You don't really adjust trim to compensate for increased weight load on the boat. You adjust trim to help keep the boat's planing angle in a good range.

jimh
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Re: 1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby jimh » Tue Feb 04, 2020 11:04 pm

Kenb wrote:Q2: how much does the tilt angle matter?


The engine trim is important, and that is why it is adjustable. Without power trim most engines provide a range of fixed down-trim positions that can be selected mechanically.

Power trim-tilt is also nice if the boat is operated by someone who is not strong enough to be able to tilt-up the engine by their own strength, such as a young person.

Kenb
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Re: 1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby Kenb » Wed Feb 05, 2020 9:29 am

Thanks for replies. Some thoughts and additional clarification.

Q1 extended version: The Mercury 25 FOURSTROKE is made by TOHATSU. I figured the best [way to get advice about a propeller] was to ask people who own that engine.

In my experience, calculating prop slip and load tends to work more poorly the smaller the hull and the stranger the bottom of the hull, e.g. a 13-footer.

My experience is that, on bigger boats with deep V-hull, the trim angles matters but not as much as on flat bottom boats. On flat-bottom boats the trim angle is critical, and [the effect of trim angle] shows up in the GPH and MPG. Outboard engines on deep V-hull boats don't need to be adjusted for trim, while [the engine trim on] flat bottom skiffs does need some tweaking, based on changing load.

And I am confused whether a classic 13-foot hull acts more like a V-hull or more like a flat bottom skiff.

My concern about tilt trim:

Q1a: when you add people or shift around people in a classic 13-foot Boston Whaler boat (as made in 1969 and 1974) with a 25-HP engine, do you have to adjust the tilt trim frequently?

Q1b: Or, does a correct tilt trim angle mostly work for all situations?

If the engine needs to be trimmed for load, then power tilt-trim is a must.

If the hull is stable under varying loads then I would rather have the manual version, where a knowledgeable person can adjust trim angle by moving the metal rod after manually tilting the motor up.

The counter to "someone who can't lift the motor" is that they are also likely not to be running at speeds where trim will matter, nor will they likely know when the boat is trimmed out correctly. In other words, for a boater--young, old or lazy--who can't lift the manual tilt/trim, a power tilt/trim might get them in even more trouble than it is worth, that is, power trim solves problems that don't exist. I've seen this all over the whaler community: consider the number and price of 20-inch transom extension solutions for sale because folks can't be bothered to buy some epoxy, saw dust, and fiberglass

Q6: Would you pay an extra $1000 for powered tilt-trim?

Q7: How valuable is being able to adjust tilt-trim on a 13 while running?

Q8: most important of all: which propeller is best?

The 13 is really light weight at less than 600-lbs, but the hull shape so much drag.

If [readers] have a TOHATSU 25-HP EFI, I believe they have the 9.5 x 9.5 propeller, [suggested for boat weight of] 1,100 to 1, 700-lbs [and boat length of] 13 to 17-feet.

Cf.; https://www.boats.net/catalog/tohatsu/2010/mfs25b-4-stroke-tohatsu/lower-unit

Part# 66: https://www.boats.net/product/tohatsu/3R0B645180?ref=1b384b92c7388502a5b3dced3a124f6e3042561e

The gear ratio is of the Mercury 25 FOURSTROKE is 1.92:1.

Cf.: https://www.mercurymarine.com/en/us/engines/outboard/fourstroke/25-25hp/

Here is the propeller suggestion table from Mercury
    9"x12.5", 600-1000lbs, up to 16
    9.25"x11", 900-1300lbs, 13' to 16
    9.5"x9.5", 1100-1700lbs, 13' to 17]
    9.75"x8", 1500lbs+, 16'+
Last edited by Kenb on Wed Feb 05, 2020 9:48 am, edited 7 times in total.

Kenb
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Re: 1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby Kenb » Wed Feb 05, 2020 9:33 am

From the table, 9 x 12.5 looks to be correct, but because the bottom of a 13-footer has so many contours and reverse cambers, I think the 9.5 x 9.5 is best--[is that] right?

jimh
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Re: 1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby jimh » Wed Feb 05, 2020 3:33 pm

What is the recommended full throttle range for the engine under discussion?

Let me guess the engine speed range will be around 5,500-RPM, which should be easy for a small four-stroke-power-cycle engine to accelerate to. With a 1.92:1 gear ratio, a 9.5-inch pitch propeller with 10-percent slip would give a boat speed of around 23-MPH at full-throttle. That sounds about right, particularly if, as your questions seem to imply, there will be several people on the boat.

With a four-stroke-power-cycle outboard engine, you probably want to pick a propeller that will let the engine accelerate to just about its maximum rated engine speed. Generally the peak horsepower output of four-stroke-power-cycle engines tends to occur very close to their maximum rated engine speed.

Perhaps some historical perspective will help you make a decision on power-trim: in 1958 when Dick Fisher was developing the 13-foot Boston Whaler boat hull, do you think he had power-trim on the outboard engine he was using?

Re the hull shape of the running surface with the hook at the transom on some older models: this hook in the hull running surface tends to hold down the bow. This was probably incorporated because engines with adjustable power-trim just were not very common in c.1959 when the boat was being developed. I believe some owners of older 13-foot hulls with a hooked bottom have experimented with fairing the hull bottom and eliminating the hook.

jimh
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Re: 1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby jimh » Wed Feb 05, 2020 4:59 pm

Consider the propeller selection problem with the Crouch's Calculator approach of first estimating the top speed likely to occur with a 25-HP engine on a boat with a total weight of 1,400-lbs and a hull factor of 180; with those parameters the Crouch method predicts 24-MPH.

Since the 9.5-pitch propeller should produce 23-MPH at 5,500-RPM, that pitch seems quite reasonable. If you can get the engine to accelerate to 6,000-RPM, you should hit 24-MPH or more, and that is about all the speed there is to be had.

In any small boat, the effect of crew weight will be a very significant factor in total boat weight. The smaller the boat, the greater effect of crew weight. For this reason, it is unlikely that one propeller can be the optimum propeller for the engine and boat in the two distinct situations where only one person is aboard and when three or four people are aboard. The difference in crew weight between those two situation is going to be as much as the hull's weight. You may have to pick the propeller based on accommodating the highest number of people to be aboard.

Also, although it has been a while since I bought a small aluminum propeller for an engine of 25-HP, I don't think they cost very much. Buying a propeller and having to relegate it to a spare because you guessed wrong about the pitch is not going to cost a fortune.

jimh
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Re: 1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby jimh » Wed Feb 05, 2020 5:18 pm

Here are several prior discussion on the topic of 13-foot hull trim. Here is an interesting one:

http://continuouswave.com/ubb/Forum4/HTML/007445.html


You can find more with a Google search using this argument:

site:continuouswave.com/ubb/Forum4 13-footer bottom hook

biggiefl
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Re: 1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby biggiefl » Fri Feb 07, 2020 10:54 am

Having owned many 13-footers I would not use a 9.5-pitch unless running four people all the time. I believe my 35-HP engine--which is probably more like 28 to 30-HP today-- was a 2:1 ratio and I ran a 10 x 13 propeller. With a light load I could run a 15-pitch. If you are running a heavy load, get a larger engine as you will not be happy with a 25-HP engine.

Power trim is very important but obviously not necessary. With it you can maximize performance--unlike with a tilt pin. I use my power trim all the time to get on plane faster and to minimize porpoising--13-footers are prone to porpoising.

Is power trim power worth $1000?

Is 25-HP enough for your needs?

TO answer we need more information to make a decision.

Are you buying from a dealer?

Have you priced Tohatsu as well?

If dealer, they usually will let you switch the propeller if it is not the right size. Without a tachometer, probably just a guess anyway.

I would ask a old school dealer who is still around that now (obviously) sells Mercury and ask them. They know this stuff. One dealer that has been around since the beginning is Hance & Smythe in Manahawkin NJ.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall. :roll:

biggiefl
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Re: 1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby biggiefl » Mon Feb 10, 2020 1:27 pm

Again, more info. is needed. What is the redline and is gear ratio correct would be a starter. I run a 17" pitch on my 13' with a 40hp Suzuki, same prop I used on my 40hp Evinrude. I think a 12" or so would be a good starting range but too many variables. I find it odd that the largest pitch offered is 12" and hence why I would call the dealer and ask.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall. :roll:

Kenb
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Re: 1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby Kenb » Mon Feb 10, 2020 9:33 pm

biggie - The whole point of posting on a whaler website was to avoid guessing, which is what the dealership I bought the engine from was doing. They didn't know which prop for classic whaler 13s, as the new whaler 13s have a lot more Vee. His suggestion was to ask online, since feedback from someone running the same set up (hull and engine) is best.

Regarding power, after reading the threads on the '13 hook,' it seems to me that anything over a 30hp EFI 4 stroke on this hull is horrible idea.

biggiefl
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Re: 1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby biggiefl » Tue Feb 11, 2020 10:36 am

You still have not answered any of my questions. If you are carrying a heavy load that will depend on what size prop and hence why I asked the question--I think twice. iIf we have the max engine RPM and gear ratio, we can put it in a propeller calculator and get a reasonable estimate of pitch as well.

The hook is there to try and limit porpoising which why I stated above it is a good reason to have PTnT.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall. :roll:

Kenb
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Re: 1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby Kenb » Tue Feb 11, 2020 12:22 pm

[The range for full throttle engine speed for the Mercury 25-HP FOURSTROKE engine is] 5000 to 6000-RPM. The gear ratio is 1.92:1.

Source: https://www.mercurymarine.com/en/us/engines/outboard/fourstroke/25-25hp/

biggiefl
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Re: 1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby biggiefl » Tue Feb 11, 2020 2:02 pm

If boat does 30-MPH and the engine accelerates to 6,000-RPM, the recommended pitch is 12.5. If running heavier loads like the people and their gear I would go for the 11-pitch.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall. :roll:

jimh
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Re: 1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby jimh » Thu Feb 13, 2020 9:11 am

Nota Bene: this thread is discussing selecting a propeller for a particular engine and boat combination. Let us stick to the topic.

jimh
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Re: 1969 SUPER SPORT 13, Mercury 25-HP FOURSTROKE; Propeller; Power Trim

Postby jimh » Fri Feb 14, 2020 10:47 am

Perhaps two propellers will be needed. One for boating alone and need maximum speed. Another for boating with three or four aboard and needing the ability to get the boat on plane. Aluminum propellers in this size are not very expensive.