The Dreaded Repower Question....

[Turner]

You can increase the horse power all you want but sounds like you have a full displacement hull. Full displacement hulls don't go fast, increasing horse power will probably cause the stern to "squat" in the water resulting in same speed and increased fuel use. Most likely never run more than half throttle. I went thru repowering my 30' Sundowner three years ago, wanted at least a 150 HP, our old one was 60 HP. Spoke with a Beta Marine rep and he went over what I just explained above, I insisted on a 150HP and he refused to sell me one! He said I'm not going to be part of you wasting your money go somewhere else to buy! He recommended a 50HP after Beta Marine calculated hull shape and prop size. He said a little over half throttle you will run at 7 knots, use 1.5 gph and can run 24 hrs a day like that. I eventually agreed with him, installed the 50 HP and he was correct on everything he told me! Saved me a bunch of money! Sometimes more is not better when it comes to HP and hull shape. The sales guy was Joe, at Beta Marine in Connecticut ( I think). Give him a call and talk to him. Most honest salesman I ever run into.l

 

[Kit_L]

Turner: I love that story about the salesman. Good on him, as we say here. And he has a customer for life, and goodwill everywhere.

 

[Bryant]

repower

So, I have a 20,000#, 36 foot Pelagic Trawler which has a deep V hull with keel and hard chines. The bow has a deep entry and the rear of the hull is fairly flat. we really like the boat but it is too slow for us. It is currently powered by a Perkins Range 4 M135. 120 HP at 2800 RPM, with 2.5: 1 reduction gear turning a 24" X 11 Prop.
At 2200 RPM we burn about 2-2.5 GPH for 7 knots. We really would like to get 8.5 with the ability to run to 12knts or better to beat the tides. We cross bars a lot so sometimes it is really nice to be able to push up the speed. It can mean the difference between crossing and waiting out an ebb.
We are looking at boats but have seen nothing in our price range that inspires us or that we like better than ours except for the speed. I was thinking or repowering with a 250 HP engine. The tanks are new, I have a 2" shaft and the engine compartment is huge, clean and open. What are some thoughts on replacement? I know of one Pelagic same hull as mine that is a seiner that has a Cummins 6bt that cruises at 8.5 and does 12.5 WOT with a full load of ice and gear. I am thinking that if I went Perkins though it would be cheaper and easier but I am not a huge fan of the turbocharged aftercooled Perkins. I really need some input from non salesman about which way to go.

Forgive me for asking a dumb question, but do you have a displacement or semi displacement hull? If it's a displacement hull then you could put a jet engine on it and it wouldn't go faster than hull speed which is roughly 1.3 x the square root of the waterline and that's in perfect conditions. Figure in wind, tide, waves and you won't even make hull speed most of the time. If it's a semi displacement hull then you can get more speed with more hp but you will notice a significant increase in fuel burn/hour.