Question about transom weight.

[cs90453]

I have a 15.5 ft 1975 Trihaul boat and recently blew the engine. I am in the market for a new one but not sure of the transom weight of my boat. The engine limitations is still stamped there and it says max 90HP but I'm sure that engines are heavier now especially the 4 strokes. My concern is that I wouldn't want to put a 90HP on it and it sink or swamp or tear off the transom. Is there a way I could find out this or anyone out there know of a place to find this out? Much appreciated because I am dying to get back out on the water again.

 

[The Famous Grouse]

Re: Question about transom weight.

You are correct in that especially today's 4 stroke outboards are considerably heavier than old 2 stroke technology. It can be as much as 20-40% heavier than a comporable-power 2 stroke, depending on model.

Unless you can find a marine surveror who's willing to go out on a limb, I think you're going to have trouble getting any answer beyond, "You should be ok with a XX HP. Probably. Maybe."

What I'd suggest is that you weigh your blown engine before you get rid of it. The best way may be to find a truck scale and do a weigh-in before you have the engine taken off and then weigh out after it's off to determine the weight.

Once you know the original engine weight, you'll at least know what should be a reasonably safe weight. From there, you may get some input or have some ideas on your own about how much you can increase the weight. You say your boat is rated for 90 max, but what do you have on it now? If it's a 75 HP, then you should have some scope to increase weight assuming, of course that the transom is in good condition. Back to the idea of a surveyor.

This may all be academic, however. Are you really even considering a 4 stroke? You certainly could, but the cost would be significant and on a 1975 boat, the engine would be worth vastly more than the boat itself, so resale (if that's even a consideration) without taking a loss would be an issue.

On the other hand, there are plenty of servicable 1980s vintage 2-strokes available at reasonable cost and many of them may be very close to the weight of your original engine.

Grouse

 

[ezmobee]

Re: Question about transom weight.

There's probably going to be some debate on this one due to the fact that motors were crankshaft rated back when your boat was built but are prop rated now etc etc etc. However, in my opinion, if your transom is in great shape, it will support 90HP. However, I would indeed be concerned about the weight of a 4 stroke. If I were you I'd try to stick with the 3 cylinder motors to try to save some weight.

 

[chriscraft254]

Re: Question about transom weight.

Just curious, what s the weight difference between a new 4 stroke and and old 90 hp 2 stroke like the op has? 20,30 pounds? I would stick with a two stroke anyway.

 

[ezmobee]

Re: Question about transom weight.

Just curious, what s the weight difference between a new 4 stroke and and old 90 hp 2 stroke like the op has? 20,30 pounds? I would stick with a two stroke anyway.

I wish cc254. I just looked, it's more like 100!


http://www.mercurymarine.com/engines/outboards/fourstrokes/75-115/ (go to specifications) 399lbs. My old 2 stroke 90 is a little under 300.

 

[The Famous Grouse]

Re: Question about transom weight.

Just curious, what s the weight difference between a new 4 stroke and and old 90 hp 2 stroke like the op has? 20,30 pounds? I would stick with a two stroke anyway.

The rule of thumb that I read in a boating magazine article 4-5 years ago was that most 4 strokes (greater than 100 HP) are 30-35% heavier (on average) than the same engine in a 2 stroke. They gave several examples from each maker that had a currrent 2 stroke and a 4 stroke in the same HP and that provided a reasonable amount of evidence to show that this figure is fairly close.

This was strictly presented as a rule of thumb and the context was calculating if you wanted "similar performance" when repowering from an old-school 2 stroke, you would (in theory) have to upsize the engine to make up for the additional weight.

I agree, I can't see a 4 stroke being viable unless the OP uses his boat a LOT and therefore can get an ROI from fuel savings. Personally, I thought about repowering my fishing boat from a 150 HP Ocean Pro carbed 2 stroke to a 4 stroke, but when I did the math there's no way that it would be viable even though I was going through fuel at the rate of 100 gallons per fishing month on average. I'd have had to have a WAC rig like yours with twin gas guzzlers like the Ocean Pro to make it even remotely justifyable.

Grouse

 

[cs90453]

Re: Question about transom weight.

Thanks for the input. I forgot to mention that it originally came with a Johnson 70hp that weighs 273lbs. The Yamaha 4 Stroke is about 375lbs and I'm assuming that all 4 strokes in that class are about the same. The Evinrude Etec is 319lbs so I'm thinking that would be the best choice. I'm thinking about looking for a used engine because i agree that the boat may not be worth a new engine on a resale standpoint. The hull and transom are in great shape and I have recently taken out the floor and replaced it with a new one and I put a layer of fiberglass on it. Also water sealed the bottom side so should be good for a while.

 

[idonthaveaname]

Re: Question about transom weight.

I had the same question a while back and did sort of what you are doing. My original 2 stroke was about 50hp and the CC sticker said power to 60HP. Since the boat was built mid 80s I looked at the weight of a mid 80s outboard and moved on from there. It's a simple calculation. So if your original weight is good for a 70HP @273lbs then an X stroke at X HP should be equal to around 300lbs. I believe it was a spot on Ship Shape that compared all the motors and Yamaha had the lightest of all 4 strokes.

 

[chriscraft254]

Re: Question about transom weight.

Thanks for the input. I forgot to mention that it originally came with a Johnson 70hp that weighs 273lbs. The Yamaha 4 Stroke is about 375lbs and I'm assuming that all 4 strokes in that class are about the same. The Evinrude Etec is 319lbs so I'm thinking that would be the best choice. I'm thinking about looking for a used engine because i agree that the boat may not be worth a new engine on a resale standpoint. The hull and transom are in great shape and I have recently taken out the floor and replaced it with a new one and I put a layer of fiberglass on it. Also water sealed the bottom side so should be good for a while.

If thats the case, I would look into the Etec 75 hp. I had a 1976 15'Mako tri-hull that use to fly with a 70hp motor. I think that is all you would need.

 

[southkogs]

Re: Question about transom weight.

My last boat was a '71 Trihull with a 65 'Rude on it (original O/B) and it would pull two skiers up with 3 people in the boat. 70 or 75 will be a nice motor on that boat.

 

[cs90453]

Re: Question about transom weight.

I actually got a good deal on a 1990 85HP yamaha 2 stroke about 5 yrs ago. Got it for $500 and it lasted that long. Didn't have power trim but I still got 33.6kts top speed and I found the 70HP was a little too under powered for that boat. If I had 3 people in it the boat didn't want to plane off unless someone moved to the bow. The 85HP had no trouble no matter how many was in the boat so I figured 80HP min.

 

[NHGuy]

Re: Question about transom weight.

If you can spend the money the e-tec is the best way IMO. It's a no mix 2 stroke without the wasted oil in the fuel. So you get the power without the weight. Unfortunately they cost real money.

 

[Bob_VT]

Re: Question about transom weight.

A 1975 boat............. Requires full inspection BEFORE any motor is mounted.

 

[oops!]

Re: Question about transom weight.

A 1975 boat............. Requires full inspection BEFORE any motor is mounted.

fully agree......

as far as the weight.......at the top of the restoration forum is a sticky.......the sticky contains uscg rules.........in there are the calculations for the weight limit of hulls.