Outboard size for 21ft boat

[jstcrzyengh]

Hello,<br /><br />I am considering the conversion from I/O to outboard. Currently there is a 245hp 305 I/O in there, but it is an old OMC and repowering to a new I/O, for space reasons, is out. I am considering buying 1 new 250hp 4 stroke outboard, or two 150hp. My question is which would be best for fuel economy and overall performance. It is a fishing boat, not trying to win any sea records <br /><br />James

 

[bigbrownbuku]

Re: Outboard size for 21ft boat

2x F115 yamahas would be about as reliable/fuel economical a package as you could get and perform pretty much the same as you had.

 

[jstcrzyengh]

Re: Outboard size for 21ft boat

The boat is kind of heavy at 21.5ft long by 8.6ft wide. I do plan on moving the fuel tanks to mid boat to give a better weight reduction, but as I understand it even dual outboards will weigh less than the current electric shift OMC setup that is in there now... It will just be shifted back on the armstrong bracket. Is 115 too little? That's a 230hp at wide open throttle. Also forgot to mention this boat has a cabin, its not a center console and is a 71.<br /><br />James

 

[Osprey]

Re: Outboard size for 21ft boat

I have a 20' Graddy White 208 cuddy cabin which has a 1995 175 Yamaha. The boat is used offshore in all kinds of open ocean conditions and this engine/boat combination performs great. When / if I repower I would not go smaller, likely would look at the 200 4-stroke.<br /><br />Cruise speed is about 18-20 knots in moderate wind/chop conditions. Fuel economy is not great but about 2 miles per gallon.

 

[rodbolt]

Re: Outboard size for 21ft boat

jstcrzy<br /> you will have to inspect the transom carefully. most the old OMC hulls had very thin weak transoms like most inboard boats. the old OMC drives transmitted thrust to the stringers. modern power plants transmit it to the transome. have the transome looked at by a qualified engineer or hull specialist. this is not the place to price shop.

 

[JB]

Re: Outboard size for 21ft boat

Outboard Non-repair topic.

 

[Solittle]

Re: Outboard size for 21ft boat

Twin 150s are probably more than you need and may be on the heavy side (twin V6s). I run twin 110 hp V4s on a heavy 23' CC and the performance is just fine for our use. 115s would be just fine.

 

[jstcrzyengh]

Re: Outboard size for 21ft boat

SoLittle,<br /><br />If I can ask you a few questions. What is your average cruise speed? What is your WOT speed? What is your GPH use at Cruise and WOT? Thanks for any and all information.

 

[b.gagnon]

Re: Outboard size for 21ft boat

rodbolt... Is right, You may have more work than the boat is worth if it is a stringer mount.

 

[Dhadley]

Re: Outboard size for 21ft boat

And dont forget the transom angle is probably different than an outboard model. Youll need to pick that up in the bracket. Most do anyway.

 

[Dhadley]

Re: Outboard size for 21ft boat

In the end a single 200 hp anybrand is going to be better than what you had.

 

[phatmanmike]

Re: Outboard size for 21ft boat

twin fourstrokes will be lotsa weight and dont forget extrra drag when under-way. <br /><br />a single 200 or 225 or 250 would be very wise