Repower but with 120 hp mercruisers?

[Tricabin73]

I have a 73 tricabin it currently has beat 360s. And I do have a strong pair of 454s I was going to repower with next year ( this years project were the decks)

So anyways. Where I boat it's mostly no wake out to the hang out beach. I'm probably never gonna go too far. So I was wondering it I put a pair of 120s. Could I prop it out to get 10 mph at 3000rpm and have maneuverability? I'm not look for planing or speed.

What you guys think? I'd like a day on the boat that doesn't kill the wallet

[comodave]

Gettaway repowered his F32 with 165 Mercruisers and had very good results if you would be satisfied with slower speeds, but very economical speeds. Do a search for the posts by him. He said he would have gone with the 120's if he had not found the 165's.

[rossjo]

If its for you, do it.

If you plan to move to anothr boat before you die then don't ... the resale will be nothing ...

[Commissionpoint]

The 36 Tri weighs a lot more than an F-32. The 165 Mercs are I-6 and make considerably more torque than a Merc 120, which is an I-4. The 120's will be under high load basically at all times due to propping requirements in order to maintain reliable and safe vessel manuverability, espescially at slow speeds. 120's havent been made in years, so the best you could hope for would be a quality rebuilt unit, and even then I think it would prematurely fail. Add all of this up and it will cost more to undertake this project than it will save in the long run on fuel by far.

If you have the big blocks already, I think the answer is simple. If not, find some decent 350's and be done with it.

+10 on what Ross said also. If you don't plan on it being yours forever, you will have a hard time giving it away later.

[Tricabin73]

Thanks guys.

I have access to 2 120hps so that's where the idea came from. I cam accosts a cool website that shows gph for single and twins at any rpm. Don't know the accuracy but it gives me insight. I still need to finish my decks so this year it will remain the Chrysler 360.

I might keep an eye out for some v6s. Or maybe sell the big blocks and start over come next winter.

[Tricabin73]

Opps. This is the websight
Also computes diesels

http://www.boat-fuel-economy.com/mercur ... ngine.html

[Kiwipushrod]

If the 120's were propped correctly, why would they wear out "prematurely"? They were awful bulletproof.

Kiwi

EDIT: That GPH site is very accurate. I would not have posted it, if not. It is also adjustable by voting.

[prowlersfish]

If your happy with under 10 knots Then go for it . But 10 knots is plowing water s you'll be burning some fuel and you'll be working the engines . at 7-8 knots you'll be sipping fuel .7 knots would be perfect

Resale ? Being it will be a fuel sipper it may not be that bad . Gettaway got a good price for that reason and the tri would be more suited for low speed boaters IMO . You would lose some folks but could gain others in the market for a cheap trawler .


As fas as the GPH site you can use it as a guide but don't bank on it . to be in the ball park you have to be propped right . It is not a calculator it just some specs .that I suspect can from the engine company's to start and changed with votes .
Example 300 hp Volvo diesel max fuel burn 14 gph yet merc diesel is almost 20 gph ?? A real number for both would be 18gph ( using a real calculator ) while the would be a very slight different between brands , it would not be 6gph
300hp @ 14 gph is a pipe dream can't happen .

Its a cool site and guide just don't bet the bank on the numbers

[Tricabin73]

It's still up in the air but how would I guess on prop size?

[prowlersfish]

Use a good prop calculator . I can check one I use . What gear ratio would you be using ? Do you happen to now what rpm the 120s are rated at ? (I am guessing 4000-4200 )

[Tricabin73]

Thanks. My options are the trans on the 360s. Or the trans that are on the 454s. Right now I have no idea of ratios on either but I can look tomorrow. Are there stamped plates indentifying them?

[prowlersfish]

There should be plates on them