The Dreaded Repower Question....

[Finnegus]

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.

 

[SteveK]

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.

 

[Finnegus]

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.

 

[SteveK]

What RPM at 9

 

[Finnegus]

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.

 

[High Wire]

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?

 

[Finnegus]

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.

 

[hobbystuff]

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.

 

[Comodave]

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!

 

[hobbystuff]

Changing the pitch a little will not get him where he wants to be.

Agreed.

 

[PM]

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.

pete

 

[Nomad Willy]

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.

 

[socalrider]

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).

 

[rgano]

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.

 

[Lepke]

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.

 

[rslifkin]

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.

 

[twistedtree]

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.

 

[SteveK]

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.

 

[DavidM]

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.

David

 

[Choices]

Lose the 1800# if possible. Or, test with low fuel to compensate.

 

[Finnegus]

Thanks for all the responses, lots to think about... I did have a chat with Vicprops and they recommended a 24×12 four blade. My current is three. He thought it might make a significant difference. The Range 4 is goverened at 2800 RPM so with no prop it wont go past that. Vicprop said the gear and shaft would handle a lot more torque than i could possibly apply. They have a reconditioned prop in house that can be repitched at a very good price so I guess I will give it a shot. They know the boat as they are fairly common as commercial vessels. He said I should have a 8-8.5 cruise and 12 WOT.

 

[SteveK]

Finnegus. Thank you for the follow up info. Do let us know if the prediction speed is achieved once you get it installed. Good info for all.

 

[Finnegus]

You are correct it is a semi displacement. I think that is where the compromise came in. The boat has an extremely shallow and flat aft section of the hull. When they used the hull for commercial use the Ice hold was forward and work deck aft so no Biggy and the boats out perform the theoretical hull speed. When this boat was built as a pleasure craft with the 120 Perkins it gave it a bow down aspect that they compensated for with the ballast.
I used to have a Mainship 34 with a Iveco 260 hp. it would do 15 knts but was squirrely as heck and felt unstable at that speed. we always cruised it at 7-8 and ran 12 if we were trying to beat a tide.

 

[Ski in NC]

A 2800rpm motor with 2.5:1 gear and 11" pitch prop is way under propped. Would not surprise me that at 2800 the engine is bouncing off the governor.

But even propped right, 120hp is not going to do much more than hull speed with 20k lb, 36'.

When I set up my 38' 14000lb boat, I was going to go with the 6BTA 370. Some friends talked me into the 6CTA 450. I resisted at first, then buckled. Got a used 450. Glad I did.

I figured I needed 200-250hp to do 20kt, and that turned out to be pretty close. The 370 could do it, but it would be working pretty hard. The 450 does 200hp at 1900 without breaking a sweat. Where I run that engine is close to where the continuous duty engines are rated to run.

So if you are going to add power, go big or go home!! I have no idea what your hull shape is like, but you are heavier and shorter, so it will take more power than I need.

No fun to chuck a 370B in there and find out the boat wants just a bit more.

The C series is a few inches bigger than the P354 and B370 in each dimension, and about 600lb heavier. You've got a 2" shaft that can handle the C with 1.7-2:1 gear. Then a near square wheel.

 

[dsharp]

Is your boat the same as the one that they have listed on yacht world? It’s definitely a hard chine with a deep keel and forefoot. Is there any way to move the engine or fuel tanks back to avoid the ballast? The 2” shaft will handle 250 hp with no problem. You may have to increase the exhaust size depending on what you have now as well as the raw water intake. What gear are you running now? Switching to a different brand engine shouldn’t be that big of a deal. You will probably have to make changes to the engine beds. I would talk to some of the boatyards in the area and see if they have done similar re-powers. We sold our yard 25 years ago so I’m not selling anything, but more power will change that boat completely.
I would be curious to see how deep that boat runs in the stern. It looks like a heavily built hull.

 

[sbman]

You are correct it is a semi displacement. I think that is where the compromise came in. The boat has an extremely shallow and flat aft section of the hull. When they used the hull for commercial use the Ice hold was forward and work deck aft so no Biggy and the boats out perform the theoretical hull speed. When this boat was built as a pleasure craft with the 120 Perkins it gave it a bow down aspect that they compensated for with the ballast.
I used to have a Mainship 34 with a Iveco 260 hp. it would do 15 knts but was squirrely as heck and felt unstable at that speed. we always cruised it at 7-8 and ran 12 if we were trying to beat a tide.

To determine the actual power produced calculating the fuel flow will tell you how much power the engine is making, if you are underpropped and limited by the governor then you'll see a fuel flow lower than the spec at max RPM. There are a variety of ways to do this from an installed fuel flow meter to measuring fuel consumed from a graduated vessel in a fixed time period. It's valuable information to obtain given your goal, once you know how much power is on the table you can make an informed decision about how to proceed.

 

[Brooksie]

Way light on pitch. At your stated 2.5 gph you are producing only 43-45HP. My Perkins 160 uses 8 gph and produces 145 hp at fast cruise. You may actually be running up against the governor although that would usually be set for 3100 or 3150. It should be marked on your injection pump or go WOT in neutral for 20 sec and see what you get. If 2600, there is your problem.


Not the full displacement boat shown but my downeast boat.

 

[rossgoldie]

Hi there ,

1. Your engine is hitting the governor at 2800 rpm. WOT full load should be about 2500 - 2600 rpm.

2. It looks like you have the commercial prop for actual trawling (towing) , where you want pulling power not speed .

Clean all your sea strainers , heat exchangers and coolers .. Keep an eye on the engine water temperature with a new prop fitted plus there should be no grey/black smoke at WOT, but you will get some steam ...

Go up to about 24 x 20 and the boat should easily get double figures . It will start to squat at the stern as it tries to climb up its own bow wave.

If so ,start by removing some the 1800lbs of ballast .. you will eventually come to a happy compromise .. of speed , fuel consumption and boat motion.. all for the price of a new prop. .... sorted !!

regards

RossG

 

[Kit_L]

Joining this thread out of curiosity, especially to see if re-pitching the current prop. makes any significant difference. Good luck!

 

[BonesD]

I’m curious why no one suggests taching the prop or the drive shaft. The transmission I imagine has technical data but who know what gears are inside the housing unless you purchased it new. Even if it’s what it’s supposed to be without the outgoing RPM you are just guessing.
Engine rpm numbers are important but what are the rear wheels (PROP)actually doing?