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Old 05-07-2021, 01:18 PM   #1
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The Dreaded Repower Question....

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.
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Old 05-07-2021, 01:35 PM   #2
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Curious, with a clean bottom, have you had it at max rpm, at least the rated 2800 for 120HP? What speed?
Often discussed that a 120HP capable engine is only using 60-80 HP at cruise, like 7Knots.
Seems to me that you could get 12KN out of it for at least short periods.
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Old 05-07-2021, 01:38 PM   #3
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What surprises me is that at WOT I am getting about 9. The bottom was striped and painted this year and the prop was reworked and balanced. I do have keel cooling but I cant imagine that the 4 tubes create that much drag.
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Old 05-07-2021, 01:43 PM   #4
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What RPM at 9
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Old 05-07-2021, 02:19 PM   #5
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I am getting 9 to barely 10 at 2800. I verified the tach on the boat and it is spot on.
The engine is is perfect shape, has had compression test and blowdown all within factory norm. I wonder a bit about the reduction gear being so low. Maybe more rpm on the prop would help??
But It feels like the engine is barely loaded even at 2.5 to 1. I have had other boats where you could tell that the engine was loaded your could hear it bog down when you throttled up. The perkins just flows through the throttle range in gear, even if done quickly with no noticeable change in pitch from the engine. The boat accelerates quickly to cruise speed.
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Old 05-07-2021, 02:29 PM   #6
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So a Monk 36 weighs about the same with the same engine gets about the same speed but with a 24X20" 3 blade prop. There are a few with higher HP single engines that will plane off up to about 15 knots with a 370 HP Cummins BTA and more prop.
Your pitch seems a little light. Will the engine go past 2800 at loaded WOT?
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Old 05-07-2021, 02:43 PM   #7
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The engine is rated to 2800 and I can get 2800 easy so maybe I should try more pitch. Be wayyyy cheaper than a repower!
The hull on my boat was designed as a seine boat so they added about 1800# of ballast under the cockpit. I am not worried about the boat squatting too much under power. It barely does even at WOT with the ballast.
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Old 05-07-2021, 04:45 PM   #8
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I agree that your pitch looks a little low. My 36' trawler with 120hp Lehman, 2.57:1 transmission has a 24x21.5 prop on it. We do 7.5 kts at 1800rpm.
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Old 05-07-2021, 04:50 PM   #9
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Changing the pitch a little will not get him where he wants to be. The 6BT engines are great and what I would look for. They have a reman program that has a warranty equal to new, I believe, and may save you some money. Have fun!
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Old 05-07-2021, 04:53 PM   #10
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Changing the pitch a little will not get him where he wants to be.
Agreed.
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Old 05-07-2021, 06:45 PM   #11
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There is a huge, expensive risk in repowering a slow boat hoping for more speed. You may get it, you may not. I think it would be worth the few thousand it may cost to have your hull checked out by a marine engineer and let him tell you if more power equals more speed.

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Old 05-07-2021, 07:21 PM   #12
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OP hasn’t got the right boat for speed.
Probably too heavy.

Finnegus wrote;
“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’ll guess this fish boat is feather light compared to your Pelagic.
I know I know you’re think’in it can’t be. But for some reason his boat w the same power as your’s goes much faster. Could it be he’s just talk’in and it’s slower than yours. Again .. gotta be some reason. Maybe your boat is bow heavy and won’t generate the lift necessary to attain 12 knots. But there’s a reason it is like it is.
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Old 05-07-2021, 09:56 PM   #13
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Yeah I agree with the previous two posts. I know the market is tight right now but for the time and $$ a re-power would take I'd think finding a vessel that was designed and powered to get over the hull speed hump from the drawing board would be massively less risky. You can confirm performance on a sea trial, *before* sinking $50k+ for very likely zero return at resale (or when you find out you can only now get to 10kts at 4x the fuel burn! Even if it takes you six months you'd come out ahead of schedule (at least the way the yards here are booked up).
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Old 05-07-2021, 10:05 PM   #14
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Quote:
Originally Posted by Finnegus View Post
The engine is rated to 2800 and I can get 2800 easy so maybe I should try more pitch. Be wayyyy cheaper than a repower!
The hull on my boat was designed as a seine boat so they added about 1800# of ballast under the cockpit. I am not worried about the boat squatting too much under power. It barely does even at WOT with the ballast.
What I was thinking. Somebody with the appropriate credentials here can probably comment on that pitch. My 12,000 pound boat with a 315 HP engine at WOT of 3800 RPM gets 22-24 MPH. Prop is 22X22.
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Old 05-07-2021, 11:41 PM   #15
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The seiner version probably has a much bigger prop for streaming a net and is probably lighter with less cabin. It's hard to beat hull speed unless the hull planes and it's usually done with a lot more hp and higher fuel burn.
Everybody with a slow boat wants to go faster. It might be cheaper to switch to a longer boat that currently goes faster.
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Old 05-08-2021, 07:56 AM   #16
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Do you have any pictures of the hull out of the water? From the description, I'd think it might be able to plane, or at least close to it, given enough power. But depending on how much keel is down there and other draggy bits, it might take a ton of power and fuel to get much more speed. And it might have a fairly low speed limit before handling gets weird. Or, it might turn out to be a decent slow planing hull that's just underpowered.
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Old 05-08-2021, 09:55 AM   #17
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I'd recommend spending some time with the BoatDiesel prop calculators. You can experiment with different prop and gearing configs, plus larger engines.


If your engine is rated for 2800 rpm and doesn't turn a bunch faster at wide open throttle, then there is no power left in the engine to give, and you have a gear ratio + prop pitch that is delivering all the engine power to the water. Just keep in mind that there are infinite combinations that will do the same thing; Bigger prop diameter, but less pitch. Higher reduction gear ration with bigger prop, etc. etc.


If you have a displacement hull, I think it's impressive that you can reach 9-10 kts. To go a kt or two faster is going to require a lot more HP, and I don't know how well the boat will handle if you push it harder. But you can experiment with all of this using the prop calculator. Try out your existing gear & prop with a 250hp engine and see what you get. Then try the same larger engine with whatever gear and prop the calculator recommends. Also keep in mind that as you go up in power, you are likely looking at a larger shaft diameter which may not even be possible.
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Old 05-08-2021, 10:12 AM   #18
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Going faster than design reminds me when we hooked up a sailboat to tow back to docks. Well all the other boats were OK with the tow RPM we had got used to using. Apparently this boat with a normal 5 knot speed buried the stern doing 7 knots and we had to slow down. Those onboard did not know their boat could go so fast.
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Old 05-08-2021, 10:23 AM   #19
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The remanned Cummins 6BTA 270 with a jacket water cooled after cooler sounds ideal for your situation. Check the prop shaft size- can it handle the torque of that engine. A data sheet is attached which shows that the torque peaks at 682 ft lbs.

The jacket water after cooler means no frequent cleaning as required by a raw water after cooler. It should be just as efficient at slow speeds as your current Perkins but will give you some speed when you need it.

From your description of your hull's form it sounds like a semi displacement hull. 12 kts from a 270 is possible with 20,000 lbs on a SD hull but may be pushing it. On my similar weight Mainship 34T the Yanmar 370 would do it comfortably but was probably pulling 200 hp at that speed. You would be running only 200 rpm off of top to do that with the 270 which is pushing it, but not unreasonably so.

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Old 05-10-2021, 04:21 PM   #20
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Lose the 1800# if possible. Or, test with low fuel to compensate.
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