Houseboat repowering

Hi , i have a 67’x 16’ 2.5’ aluminum houseboat with bad outdrives and non supported engines. I am planning on 250hp outboards to retrofit. I would like to ask if anyone could recommend if they would put them in the exact place of the existing outdrives? Also i am concerned of the length of distance to helm which is approximately 55’

 

Pictures would help here.
Outboards should be mounted such that they can be tilted up fully out of the water when not in use. Outboard brackets are the easy way to insure room for that.
Be aware that large, heavy boats are sometimes not well mated with outboards, as a deep gear ratio and a large diameter propeller may be needed.
I don’t think that modern outboards with electronic controls require cables, which are painful in long lengths.

 

Need a lot more information, and pictures to make any guesses about putting 500hp and a thousand pounds somewhere.

 

Kapn d brings up some good points. I'll assume your having someone competent do the transom work.

Stick to fly by wire and you should be fine. Even if you go for a mechanical shift, there are electronic controls that can fly by wire convert, they are several grand but 2x 250 hp outboards is 40k these days so a few grand is just a % of total.

The issue with outboards on big heavy boats is not trivial. The industry has done a better job in the last few years with proper pitched props for slower and heavier loads but with a fixed diameter there is only so much that can be done.

On the west coast we drive heavier slower boats compared to the faster and lighter gulf and east coast Florida boats. More and more power but still 17 inch props. Sadly the numbers definitely favor the lighter faster boats of Florida. So we don't yet have a great option for maximum oomph in an outboard.

The notable exception in that range is the duo prop suzuki. It was widely adopted here for our heavier boats. The initial 350s were abysmal failures on the power head front but the guys did like the duo prop. Suzuki has launched a 300 duo prop that suppose to be an 87 octane power head like the much lover 300ap with the duo prop from the df350. I've not heard much feedback on it, as many of my friends who were burned by the df350 repowered back to ap 300s or went big to the Yamaha 425.

While you want 250s, my old Honda 225 is heavier than a 300ap suzuki and only 20 pounds lighter than the new duo prop. Might be worth considering the duo outboatd to get a little more propellar blade surface area.

 

Few 55' houseboats in our marina would be able to benefit by 500 hp. Well, none.

 

I will second (or third perhaps) a request for some photos of your houseboat, including the transom where the existing outdrives are (or were).

Dare I ask what your maximum budget is for this conversion?
If you can afford more than the cost of a couple of big new petrol outboards, then a rather 'left field' suggestion could be to fit a pair of high thrust water jets - but then you would need new engines as well.
Here is a link to a thread on here from 9 years ago, asking about Nam Jets and Traktor Jets -
Namjet, Traktor Jet https://www.boatdesign.net/threads/namjet-traktor-jet.51287/

A brief article extolling the virtues of high thrust jets -
Traktor Jet is designed for slower speeds https://www.rivieramm.com/news-content-hub/news-content-hub/traktor-jet-is-designed-for-slower-speeds-49702

I get the impression that Nam Jet have taken over Traktor Jet (?) - and all of the Nam jets available seem to be rather large.
NAMJet https://namjet.com/

Another left field suggestion - have you considered a pair of smaller, higher thrust diesel outboards?

 

As an owner of as well as around Traktor and namjet products as well as the hill innovations jets and thrust master hi500 jets I'd vote against them. The smallest namjet is 44k... then you still need an engine and gear. The thrustmaster and hi jets are near 65k. All were designed by the same guy with various falling outs from various manufacturers. While a necessary evil for me, not an ideal solution if props or obs can work.

While better for heavy applications than alamarin, ultra and Hamilton jets. They are expensive to buy and repair, and as a low volume item lots of changes in process mean tough long term parts and custom repair. Had one apart fixing it last week and the owner was joking how he'd rather learn to fish another species than keep fixing his jet.

Diesel obs are cool, wish they would have made them with a bigger prop and a deeper leg... but that's just the way the markets gone. Up here the oxe was 55k but is now over 60... so just 3x the cost of a suzuki 300ap. Little more torque and range... but even the commercial ob guys can buy a lot of gas for 40k...