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Author Topic:   23 Conquest Repower; Maximum rating formula
tim2me posted 10-30-2002 08:40 PM ET (US)   Profile for tim2me   Send Email to tim2me  
I am thinking of repowering [a 23-CONQUEST with twin Suzuki 140-HP 4-stroke engines]. Does anyone have any thoughts?
homey posted 10-30-2002 08:56 PM ET (US)     Profile for homey  Send Email to homey     
I am also considering repowering my 25-Outrage with the Suzuki 140's. I've heard nothing but good things about these motors.
bsmotril posted 10-30-2002 09:21 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
I think the Conquest 23 (I have one) is a bit much for a pair of four cylinder engines. It is a heavy boat rigged out and [it will] benefits from cubic inches. A pair of V-6 would have the grunt in the midrange to keep that hull on plane at low to mid range speeds. What power is on the boat now?
JBCornwell posted 10-30-2002 09:36 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
I don't think the number of cylinders matters.

I do think displacement matters, and a 4 cylinder will probably be lighter than a 6 cylinder of the same displacement.

Usually, more cylinders will be smoother, but if that JohnnyUki DF140 got any smoother people would think it was an electric.

Red sky at night. . .

TRAFFICLAWYER posted 10-30-2002 10:14 PM ET (US)     Profile for TRAFFICLAWYER    
I think with twin suzukis, you're gonna be seriously underpowered, maybe twin 200 Yamaha HPDI's.
tully_mars posted 10-30-2002 10:34 PM ET (US)     Profile for tully_mars  Send Email to tully_mars     
I think it is close and a great idea, but 140's aren't enough. I don't see how a 4-stroke can generate the torque of V-6 150's or V-6 135's two strokes. If I had any less power I wouldn't be as pleased because the real need for power is in a rough sea. Any less performance and it would take away from current perfection in the boat.

I have already seen a bunch of SKA fisherman dumping their Yamaha 225 4-strokes to go back to DFI motors because of the lack of get-up an go.

Tully Mars

David Pendleton posted 10-30-2002 10:47 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
I'm curious what kind of performance you outboard guys get.

My boat is a Mercruiser 5.7L EFI (260HP) I/O, Bravo III, 125gal/20gal fuel/water configuration and I can manage about 40MPH in good water.

What kind, hp, and quantity of outboards are you running?

What is your top speed?


tim2me posted 10-31-2002 09:00 AM ET (US)     Profile for tim2me  Send Email to tim2me     
Right now, I have a single 250 1998 merc EFI with a top end speed up near 50 mph. It has a weight of 485 lbs. The conquest appears bow heavy the way it sits in the water so I figure it was designed for twins. The boat is rated for 300 hp. Ive been looking at the Suzuki's and they appear to be better technology than the Yamaha & honda's. They weigh in at 415 lbs each. What do you guys think?
VMG posted 10-31-2002 09:14 AM ET (US)     Profile for VMG  Send Email to VMG     

While it's an apples to oranges comparison, 2 Yamaha F115s will push a rigged Regulator 23 nearly 43 mph (see the Regulator perf charts). I've gotta believe that 280 ponies will do just fine on a C23.

That being said, I'd find somebody who has a couple of 4 strokes and see if they're happy with the torque response curve for their rig. But, I wouldn't take anyone's word for it, I'd get a ride. Four strokes are different animals when it come to power response -- some folks like 'em, some don't.

VMG posted 10-31-2002 09:17 AM ET (US)     Profile for VMG  Send Email to VMG     
Besides, Suz 140s are 2s aren't they?
tim2me posted 10-31-2002 11:19 AM ET (US)     Profile for tim2me  Send Email to tim2me     
they came out with the 4 strokes last year. They are about 40 lmbs lighter that the 130 Honda and smaller to. The performance curves are on the suzuki site on a comperable weight boat ~ 3600 lbs.
lhg posted 10-31-2002 04:07 PM ET (US)     Profile for lhg    
If you can stand the look of that 140 Suzuki, with that crazy looking cover, they might perform just fine. And they are the same color as what you are used to.

You didn't tell us what your budget is. But I agree, the bow heavy 23 Conquest should be rigged with twin engines. Do you want new or old technology?

To get close to what you are accustomed to with the single 250 EFI, you will need two stroke 150's, DFI, EFI or carbed. Merc's 150's are all stong engines, since they are simply detuned 2.5 liter 200's. I'd go for the trouble free EFI's, but you can save another 3200 ($15,000 vs $18,200)by settling for the carbed versions! That's hard to resist.

bsmotril posted 10-31-2002 08:31 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
My conquest has the hardtop, and all other options except the anchor windlass. Equipped with a pair of Opti 135s, I get a top speed (1/2 tank, 2 people of 45mph (GPS), and about 42-43 mph loaded. It will stay on plane down to 2800 rpm and 18mph, and cruise at 3mpg at 32-3400 rpm, 8gph at 24mph in smooth seas. With two people and half a tank, it has enough grunt to plane out on a single engine, but will only turn up to about 4400 rpm in that mode. Acceleration is brisk. Because the same engine block is used by Merc for engines all the way up to 175, I figure it is not highly stressed and will hopefully live a long time.
diveorfish posted 11-01-2002 01:09 PM ET (US)     Profile for diveorfish  Send Email to diveorfish     
The 23 Conquest is rated for 300hp. If money is no object, it might be best to go for as much hp as possible if you opt for twins because of the questions Iím asking below.

I would like to throw these questions out to the group: Is the only advantage of twins is to give you a safety margin? Say you have a 23 Conquest with twin 135ís. If one fails and you CANíT get on plane with the other one, are you just left with, in essence, one huge kicker? If you are going to have twins, shouldnít each twin be able to get you on plane when the boat is loaded with people, gear and fuel, as would be the case in a typical fishing trip? Otherwise, would a big 250hp single outboard with a 15hp kicker be a better choice, as opposed to say twin 135ís or twin 140ís as tim2me is considering.

Currently I have a 23 Outrage with twin 135 Optiís. When the boat is loaded for a typical fishing trip, it wonít plane on one engine. I wonder if a 150 would be enough or would it take a 175 to bring it on plane? If it took a 175, could you hang twin 175ís, or better, on a 23 Conquest/Outrage even though itís rated at 300 max hp?

lhg posted 11-01-2002 03:46 PM ET (US)     Profile for lhg    
One of the problems in getting home quick with only one of two engines running, is the prop pitch. Generally, twin engines require a pitch 4" higher than the same engine in a single installation. So, you are left with a badly over-pitched situation running on one. This alone can prevent planing off the boat, even though technically the one engine has enough HP to do the job.

There are two solutions, assuming that under offshore conditions, etc, you can't easily change a prop underway.

1. ( I think this is not the best if CR is involved). You can carry an aluminum spare, 4" lower in pitch. This can at least prevent a lost vacation or boating weekend if you can get to a place to change the prop. Then you can zip around with one engine as long as you like. With CR, this becomes a 50% guess as to which engine will go out. Not good, or carry two spares.

2. Overpower the boat such that one engine, in spite of the high pitch props, will still plane off the boat. You still can't run it wide open, or you risk damaging the engine. but you can get by.

There also is not a big difference in cost between a big single/kicker and twin engines.
Euro transom Whalers don't seem well suited for a kicker installation.

Regarding HP on the 23 Conquest, this boat weighs about the same as a Classic 25 Whaler Drive, so why not use twin 200's on it, like the 25 can handle? A pair of 200's carbed is about $18,000.

bsmotril posted 11-01-2002 04:59 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
Why do I like twins over a single? Well, the sweet sound of sync'd up V-6s for one. The surface area of two props carries the boat much better than a single. Instant acceleration, no prop slippage. Shut one engine down for trolling giving you a greater range in trolling speed than a single. Easier to dock, differential power works nicely on the conquest. Putting the boat on a trailer in a cross current is a lot easier. Crab in with the down current engine, then a shot of reverse just before the trailer swings the bow around and lines it up. Dual electrical systems. I can run either or both batteries of off either engine. Even if you can't get fully on plane with 1 of two running, you still can often get enough speed running down seash to get the boat surfing down the seas. That will speed you up quite a bit. I also believe that smaller horsepower motors tend to live a bit longer than the big brutes. Plus, we call or boats "She", and women are nicely symmetrical.
diveorfish posted 11-01-2002 05:14 PM ET (US)     Profile for diveorfish  Send Email to diveorfish     
Lhg: That was good information. Now I need a pep talk. The reason I know my fully loaded boat canít plane with one 135 Opti is because I threw an injector about 3 weeks ago and messed up the piston (is that what is referred to as ďblowing the power head?Ē) Anyway, my dealer put on a new power head and also installed new injector sets on both engines because the 2003 versions are apparently better than the 2001 versions. (Which my engines are) I was pretty disappointed but all is fixed now and Iím ready to fish once again. Also everything was covered under warrantee. I always heard that the Merc Opti 135ís were excellent engines. I never heard of Opti 135ís having problems. Please tell me that my mishap an aberration?
diveorfish posted 11-01-2002 05:27 PM ET (US)     Profile for diveorfish  Send Email to diveorfish     
bsmotril: You make a compelling argument for twins. It also seems like counter rotating twins naturally track straighter with lees list or is that just my imagination?
VMG posted 11-06-2002 10:25 AM ET (US)     Profile for VMG  Send Email to VMG     

You wouldn't get anybody to insure the thing if you exceeded the max hp recommendation.

jimh posted 11-06-2002 01:14 PM ET (US)     Profile for jimh  Send Email to jimh     
VMG--your statement about one's inability to get insurance on a boat that is powered in excess of the manufacturer's rating sticker is not correct. There are several participants in the forum who have boats powered with more horsepower than the rating plate and have insurance.

The rating plate horsepower is derived from a formula. If the manufacturer computes a rating, he is required to use the formula. The rating is not determined by precise structural analysis, but by a fairly simple computation of length and width and other factors.

There is no Coast Guard prohibition against powering a boat with more horsepower than the rating sticker.

prj posted 11-06-2002 01:40 PM ET (US)     Profile for prj  Send Email to prj     
jimh: Very interesting and relevant second paragraph that would appear to put to sleep the notion that BW is undertagging the M170 due to a lack of 115HP availability.

However, you do state that "if the mfg COMPUTES a rating, he is required to use the formula."

Does this language imply that the manufacturer CAN do other than "compute" the rating? Say, just state any old rating that they desire to use?

That paragraph would appear to dispell the long held belief that Brunswick is understating the max. rating to move their slower selling 90s.

Please clarify.

TheSkipper posted 11-06-2002 02:56 PM ET (US)     Profile for TheSkipper    
Capacity/horsepower plates are only required in monohull boats of 20 feet or less. Even when a boat is equipped with capacity/horsepower plate, there is no express federal (i.e., Coast Guard) prohibition against exceeding the maximum horsepower listed on the capacity plate. According to the Coast Guard, however, some states have laws prohibiting this practice (see

On a side note, the formula used for figuring the maximum horsepower on boats under 20 feet, along with numerous other regulations concerning boats are available free online (HYPERLINK) .

The direct link to the formula in pdf format is (HYPERLINK)

(Please note that this page loaded very slowly for me, even with a high speed connection.)

TheSkipper posted 11-06-2002 02:58 PM ET (US)     Profile for TheSkipper    
Oh yeah, I forgot the most important thing . . . read your insurance policy very closely before exceeding the horsepower listed on the capacity plate.
jimh posted 11-06-2002 07:33 PM ET (US)     Profile for jimh  Send Email to jimh     
Skipper--thanks for those links.

What I interpret the regulations to say (and this is not necessarily an expert, informed opinion) is that if the manufacturer applies a rating label he had to compute the rating according to the official formula provided.

In other words, a manufacturer cannot apply a rating label that contains a "maximum horsepower" rating that was derived by anything other than the published formula.

You could build a boat with exotic high-strength materials and design it to take 400-HP, but your rating lablel will have to reflect the horsepower calculated with the official forumala.

Peter posted 11-06-2002 07:48 PM ET (US)     Profile for Peter  Send Email to Peter     
The formula specified by 33 CFR 183.53 to which Jim refers to is merely for calculating the maximum horsepower capacity that may be marked on the boat. This regulation does not prohibit a manufacturer from stating as a maximum capacity a number which is less than the computed maximum under the formula or the testing methods specified.
For example, if I have applied the formula correctly, the maximum capacity for the Montauk 170 comes out to about 145hp. For the classic Montauk it is about 115hp. In both Whalers, the stated maximum capacity is less than the computed maximum. I believe that Whaler has always understated relative to the computed maximum. This is probably an industry norm dictated by some prudence. However, in the Montauk 170, the understatement is pretty significant (compare its 62 percent of the computed maximum versus 87 percent for the classic Montauk or 85 percent for the classic 18 Outrage). Seems hard to believe that a bigger/heavier Montauk can't handle as much power has its predecessor in total or in percentage. Is there something wrong with the Montauk 170 hull design such that it is only safe with 62 percent of the computed maximum? I doubt it. Thus, I don't believe that Jim's second paragraph dispells any belief, common or otherwise, that the Montauk 170 is undertagged for purposes not related to safety.
TheSkipper posted 11-06-2002 08:02 PM ET (US)     Profile for TheSkipper    
Peter, it appears that you may be using the wrong formula. On the Montauk, the calculations are as follows: Length = 17 ft., Transom Width (based on beam listed on BW website) = 6.83 ft.

17 x 6.83 = 116.11 x .8 = 92.888 - 25 = 67.888. This number can be rounded up to the nearest increment of 5, so the maximum horsepower is 70.

Since the actual maximum horsepower is listed as 90, I would guess that the Length and the Transom Width are slightly higher than the numbers I used, which are the ones reported on BW's website.

Peter posted 11-06-2002 08:34 PM ET (US)     Profile for Peter  Send Email to Peter     

With all due respect, I believe that you have not used the correct formula. Check the table again because there are several formulas stated, one is for hulls with remote steering, the others are for hulls without. The answer of 70 hp does not make any sense when Whaler states a maximum of 90 hp for the Montauk 170. While Whaler might state a capacity that is underrated, it would be in violation of the regulation if it stated a capacity that exceeded the maximum as calculated by the formula. I highly doubt that Whaler, or any boat manufacturer, would do that.

TheSkipper posted 11-06-2002 08:47 PM ET (US)     Profile for TheSkipper    
Peter, it appears that you are correct. For whatever reason, I just assumed the transom height on the Montauk was less than 20 inches. The BW website, however, lists the transom height as 20 inches. Using the formula for transom heights of 20 inches or more, the maximum horsepower should be 145.
jimh posted 11-06-2002 11:37 PM ET (US)     Profile for jimh  Send Email to jimh     
Very interesting discussion re the 170 MONTUAK's power rating.

As for SUZUKI engines, they hardly show up in my local area (SE Michigan), but I saw a very big presence of Suzuki in coastal North Carolina when down there last month. Most of the engines we saw were the big 200-HP models.

The 140-HP is a highly tuned version of their 90/115 block. It is one of the best 4-stroke engines in terms of horsepower-to-weight ratio.

jimh posted 11-06-2002 11:48 PM ET (US)     Profile for jimh  Send Email to jimh     
[Changed TOPIC to include interesting second subject that developed in the discussion; was "23 conquest w/twin suzuki 140's"--jimh]
VMG posted 11-07-2002 07:44 AM ET (US)     Profile for VMG  Send Email to VMG     
Jim, I stand corrected.

Some years ago I thought about repowering a 15' Super Sport with a 90hp motor of roughly the same weight as the 70 it had. I never made the switch, in part because I couldn't find a company to insure the thing. With a few more miles on the personal odometer since then, I no longer have the desire to go 60mph on the water...;)

captbone posted 11-07-2002 11:40 AM ET (US)     Profile for captbone  Send Email to captbone     
I still think that they do it to keep price and top speed down, but on the new boats mainly price!

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