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Author Topic:   23 CONQUEST Upgrade to Twins and Over-power
jezza posted 03-17-2015 03:40 AM ET (US)   Profile for jezza   Send Email to jezza  
I have a 2001 Boston Whaler 23 CONQUEST with a Mercury 225-HP OptiMax XXL. [The] top boat speed is around 27-nautical-miles-per-hour, and I make many 200-nautical mile trips. I want to upgrade to a twin engine setup. Would it be best to change to two 25-inch shafts or get another 30-inch shaft?

The 23 CONQUEST is rated to 300-HP. I would need to buy two 150-HP outboard engine. Does anybody see a problem with just putting on another 225-HP OptiMax? This would be the cheaper option and my insurance company in Australia has told me, if a marine expert passes on it structurally, they would be okay with it.

The weight difference of a twin Yamaha 150-HP four-cycle engine setup and the twin 225-HP OptiMax seems small. I appreciate any advice on this. Cheers--Jezza

jimh posted 03-17-2015 08:19 AM ET (US)     Profile for jimh  Send Email to jimh     
I don't think you can get a new Mercury OptiMax engine at 225-HP any longer, at least not in the USA. They only make a 200-HP and a 250-HP. If you wanted a 225-HP you'd have to get a ProXS engine.

Assuming you find another 225-HP to match the 2001 OptiMax, I see three problems:

--the power will be above the rated power. This may not be as much a concern in Australia as in the USA. If you have the proper insurance and the boat is operated in a jurisdiction that does not regulate outboard power, you may be able to ignore this concern.

--the weight of twin engines. In the 2001 catalogue, Boston Whaler did not specify a maximum weight for the engines of a 23 CONQUEST. They did show the boat equipped with twin OptiMax 150-HP engines. Those engines probably weighed about 450-lbs (205-kg) each, or about 900-lbs on the transom. If you can find a qualified or certified naval architect to endorse a higher engine weight, you may eliminate the insurance and regulatory concern (above), but you will still have to live with added weight on the transom.

--the use of two 30-inch shaft engines will be a bit awkward. You'll need to invest in some sort of jack plate to raise them. This will add weight to the transom, move the engines back, and increase the lever arm of that weight, both on the transom itself and on the whole boat's trim.

I agree with your estimation that buying one more 225-HP will be less expensive than buying two new 150-HP engines. If cost is the main factor--it often is--then the decision is clear.

Peter posted 03-17-2015 04:21 PM ET (US)     Profile for Peter  Send Email to Peter     
I believe that the 23 Conquest had about 400 lbs of lead ballast installed in the transom when it came from the factory with a single outboard. If you were to repower with twin outboards, you would probably need to remove the lead ballast assuming it is there.
jimh posted 03-18-2015 07:03 AM ET (US)     Profile for jimh  Send Email to jimh     
Peter--that is fabulously good information.
Teak Oil posted 03-18-2015 08:36 PM ET (US)     Profile for Teak Oil  Send Email to Teak Oil     
Sell the 225 and go for twin 150's or a single 300. Running 2 30" engines will be very inefficient and the resale will be terrible with that setup.

I am not sure why you only want Yamaha 150's, twin Mercury four stroke 150's would be cheaper and have better performance.

jezza posted 03-18-2015 11:02 PM ET (US)     Profile for jezza  Send Email to jezza     
[To Peter:] Thanks. [Says he] will look into finding that lead ballast.
jimh posted 03-20-2015 10:02 AM ET (US)     Profile for jimh  Send Email to jimh     
There are quite a few prior articles that mention the use of a lead ballast or lead ballast kit in a Boston Whaler 23 CONQUEST when rigged with a single engine or a single engine and the hard top option. The lead ballast kit is needed to correct the bow-heavy static trim that results with only one engine or one engine and a hard top. See some of the threads in this search result listing for more information:

power2boat posted 03-25-2015 05:19 PM ET (US)     Profile for power2boat  Send Email to power2boat     
I have a [a 1995 Boston Whaler OUTRAGE 24] with twin Yamaha 115-HP engines. It was just a hull when I got it, but it did have twin outboards at one time before. I built a forward full cab and fuel tanks above deck, also forward. I removed the trim tabs, which were used to keep the bow down. It does ride bow high and I compensated by putting Doel-Fins on each engine. With twin engine with fins there is a lot of control with bow trim and [side-to-side] trim.

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