Underpropped?

[tedted]

Hi all, I had a chance to try out our West Indian 36 Trawler last week and came away feeling something isn't quite right. The boat was repowered from a 135 hp Perkins 6cyl to a 240 hp Perkins turbo 6cyl.
The prop is allegedly a 23" pitch and 21" diameter. The transmission is a Borg Warner 2.1 to 1.

At 2400 rpm the gps showed a speed of 7.2 mph. I didn't have time to set it up for knots. The engine seems to be way underworked. I didn't take the throttle up higher as I'm still trying to get a feel for things. I think it would easily go over max rpm. The exhaust temp at sustained 2400 rpm was 500 deg. I don't know if this is low but to me it seems to be.

I need to get the boat home before I can tear into this but thought I throw this out there for some input. I was curious what would be a good cruise rpm to run the Perkins 6.3544 turbo. According to Perkins the engine is rated at 2400rpm max. What's scary is this is where the former owner ran the engine all the time and suggested that I should run there also. It hasn't seemed to hurt it though.....

All thoughts would be appreciated. Thanks, Ted

 

[O C Diver]

While you may be under propped, it's not as simple as just adding more pitch. The boat will most likely not get up on plane with that motor. The object is to pitch the prop so that it turns rated RPM at wide open throttle. The maximum speed for that boat with that motor might only be 8 to 9 knots. Changing the pitch to 25 might be too much. This is an area where it's best to talk with a good prop shop that has experience with displacement hull trawlers.

Ted

 

[River Cruiser]

1st thing I would do is verify the accuracy of the tach before doing anything else.


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[Baker]

1st thing I would do is verify the accuracy of the tach before doing anything else.


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That is a correct statement!!! If your tach is correct, and you can go way over 2400 RPMs, then running it there shouldn't hurt anything if you are severely under propped. You also need to make sure you weren't running in a wicked current that was robbing you of ground speed...

 

[DavidM]

All of the above advice is good, but also do the following:


Get a phototach. They are under $50 on eBay or Amazon.


In neutral, open the throttle wide open (it won't hurt anything, the governor will limit rpm) and measure the rpm. Should be somewhere near 2,600 if the rated max hp rpm is 2,400 for that engine.


Then go out and run up to wot and measure the rpm with the phototach again (or apply a correction factor to your dash tach).


If you are much over 2,400 rpm (like 2,500+) then you are over propped and a pitch change might be indicated. Talk to a good prop shop with the above data in hand.


BTW, 500 deg EGT is low which seems to confirm that you are under propped.


David

 

[jleonard]

And don't be afraid to run it to wide open throttle (WOT) under load to see what she'll do. That won't hurt anything esp if your running only 500 deg F egt. At that temp that engine isn't even breaking a sweat.
Max you want to run at WOT is likely somewhere around 900 F.

 

[gaston]

question is the hull and prop clean ?????

 

[kulas44]

Fouled prop and hull usually increase load/ decrease rpm. EGT indicates the engine is very lightly loaded.

 

[Phil Fill]

The boat is over powered not under propped. The eagle is 58 ft, 38 ton powered by a single Dd 671, 165 hp natural. The previous engine was probably sized correctly. However its better to be under propped than over.

 

[kulas44]

Depends on what he wants. If he wants to go 20 mph the boat is under powered. Some folks here tend to think any boat with more installed power than absolutely neccessary to achieve hull speed is "over powered". I would agree, to a point, that a FD boat cant make use of much more power than is neccessary to reach hull speed, and maybe a bit faster. I'm not familiar with the hull type on this boat, but most boats on here are SD or planing hulls. If the boat in question is a SD design, repropping may get him up in the mid to high teens, and still not be overloaded. And, if he wants to go slow he can just pull back on the red knob

 

[Nomad Willy]

This is an engine load question .... speed has nothing to do w it.

I would advance the throttle slowly to rated rpm, max rpm or govonor limited rpm. If your engine is rated at 2400rpm I would suspect that the govonor is not working right if you can slowly work up past 2700rpm. See a mechanic about adjusting the govonor. Of course you'd have to be way underpropped to be able to attain 2700rpm. The governed speed should be 2 or 300rpm above rated rpm. Of course usually one checks goverened speed limit in nutral gear with no load at all.

With the governor working normally go for WOT in gear. Normal boat weight and bottom hull cleanliness should produce rated rpm or 50 to 75rpm more than rated rpm. Then basically you can't hurt your engine w too much throttle unless you run it above rated load (usually expressed in rpm re a specific engine). A lot of engines are rated for WOT at rated rpm for one hour. Many are rated 1-300rpm below rated rpm as long as you like or for some time limit. Consult your engine manufacturer or owner's manual. With an old engine no longer made you may need to find another boat owner w a manual for your engine. Most of us propped to rated rpm run from about 500 to 1000rpm below rated rpm. I run 700 down at cruise most all the time. 1000rpm down is underloaded for most engines.

Re pitch and dia pitch is the easiest to change but is limited to 2 or 3" depending on prop size and your prop man needs to be able to tell if the pitch has already been changed.
Dia can be easily changed too by having a prop man grind off some of the blade tips. But just grinding off some of the LE or TE changes mostly the blade area. I'm almost 100rpm overpropped and would like to pick up about 125rpm but my prop has already had a 1" pitch reduction. I would not like to loose any dia so reducing blade area from the LE and TE would probably be best. As to whether cutting the LE or TE .. just a guess but I'd prefer the LE (but that could be a tad more grinding) but probably cutting both edges would be best.

When it comes to this prop stuff I'd rather have a trailerable OB boat. I do and think I should go to a 4 blade w less pitch and less dia also.

 

[kulas44]

Speed has everything to do with it. He has more power than speed. He is propped for a lower output engine and now has a 240 hp engine. He needs more prop so as to obtain a higher speed (more load) with his given power. Unless he ONLY wants to run at 7.2 mph and 2400 rpm. Thats crazy. He could well make 7.2 mph at 1400 rpm and still not be overloaded. With the attendant fuel economy increase from less high rpm parasitic losses.

 

[Nomad Willy]

kulas,
The boat seems very overpowered so the only good solution here is to repower w the right amount of power for this boat.
Speed? The boat was designed for "X" amount of power to attain or maintain a certain speed. The speed consideration is (or should be) in the boat design.

But yes if the boat is a bad design bad powering methods may limp the boat along ..... for how long is anyone's guess.
But the only real good solution is matching the power to the hull.

I see my edit function is still open.
tedted you list the prop as having 23" of pitch and 21" dia. On this type of boat more pitch (in inches) that dia is very unusual and should be very inefficient. Is the prop actually a 23" dia .. 21" pitch? Most trawlers have more reduction typically 2.5-1. Does anyone here have a similar power/boat/prop and gear ratio? Is tedted's setup anywhere near typical?

 

[jleonard]

kulas,
The boat seems very overpowered so the only good solution here is to repower w the right amount of power for this boat.
Speed? The boat was designed for "X" amount of power to attain or maintain a certain speed. The speed consideration is (or should be) in the boat design.


I see my edit function is still open.
tedted you list the prop as having 23" of pitch and 21" dia. On this type of boat more pitch (in inches) that dia is very unusual and should be very inefficient. Is the prop actually a 23" dia .. 21" pitch? Most trawlers have more reduction typically 2.5-1. Does anyone here have a similar power/boat/prop and gear ratio? Is tedted's setup anywhere near typical?

I repowered an old 34 Mainship (semi planning hull) several years ago and dealt with a similar situation. I went from the original 160 Perkins to a 270 hp Cummins 6BTA.
I actually changed the tranny ratio from 2.1:1 to 1.5 1.
Prop was above square at 20 x 21.
It made a whole new boat. I could cruise at 7.5 knots and get much better MPG than with the old engine, yet I could also cruise at 12 knots (on plane)and get the same MPG as with the old engine at 7.5.
Plus for those times necessary I could cruise at 15.5 knots all day long (with an almost totally flat wake).
Overpowered? No way, just powered enough to make a very versatile cruiser.

 

[psneeld]

Good point Jay.

Some hulls are very versatile in speed.

Many here have seen that running semi displacement almost planing hulls are pretty efficient when run slowly. Obviously never efficient as low resistance hulls.

But without the power, their ability to make those 15 knots when wanted are imposible.

As long as the engine is in....and the hull could make better speed....it would seem better to fox the problem rather than repower.

 

[kulas44]

Repowering would be about the dumbest thing he could possibly do. Just prop the boat to match the installed power. And just what is the "right" amount of power for a SD hull ? Not everybody wants a slow boat. Why would it be a "bad" design ? How would a 240 hp 6-354 turning 2400 rpm at 7.5 knots with 500 degree egt be "limping along" ? These engines run like that for 10s of thousands of hours in agricultural and industrial applications. No harm done. Its just not as efficient as it could be.

 

[Nomad Willy]

kulas44,
You're right of course. I read the OP wrong and now see the boat is definitely not overpowered. Don't know the boat but it must be a typical trawler and it could very well be underpowered. The 36 and especially the 42 GB's are underpowered but if I was to go shopping for one I'd not even look at the twins .... that are appropriately powered.

But choosing props to load the engine correctly is the same exactly. The resulting speed of course will be different but the matching and tuning procedure will be the same. Power and the boat will decide the speed. And a correctly matched prop will deliver maximum speed.

Sorry about my over powered remark. Repowering re hp would be stupid .. unless the owner wanted more speed and the hull design was/is appropraite for the power and speed sought.

 

[kulas44]

Yes, you are correct "power and the boat will decide the speed". The prop just does the work.

 

[bayview]

In answer to the OP.
Running under loaded is not likely to harm anything. Engines don't fail from being underworked. So IMO you can take it home then decide if you want to use the extra power to gain speed, if that is possible with your hull design.

 

[tedted]

Sorry I haven't replied in a while. things have been a bit busy. I suspect that more load on the engine wouldn't hurt anything. I'm more concerned about running at 2400rpm forever and doing only 6.5 knots. That seems to make no sense to me. With almost double the hp as the original engine I should be able to run along at 9 knots wide open. The hull is a semi planing. Very little deadrise. I still haven't gotten the boat back home. Winter just isn't giving up this year and I don't want to cross an angry Lake Erie with a boat that probably needs more attention than I am even aware of yet. Thanks for your input, Ted

 

[FF]

I would go for a ride and with the engine warm and a GPS that reads speed create a graph from 100RPM on up .

Take at least 2 min at each RPM for the vessel to stabilize.

At some point you will add RPM and hardly any boat speed.

This would be your go home RPM , as anything above that simply is heating the lake.

To actually go faster may require a larger prop or a different transmission.

If course if the boat is a pure displacement boat you basically will NEVER go much faster.

 

[boatpoker]

37' full displacement, 165hp Perkins turbo, 1.92:1, 8knots at 1600rpm, 11knots at 2400rpm (ugly and noisy with major squat) ..... 24X20 prop.

Your prop is seriously wrong if your numbers are correct. Suggest you double check prop size and run one of the on-line prop calculators.

 

[sunchaser]

Sorry I haven't replied in a while. things have been a bit busy. I suspect that more load on the engine wouldn't hurt anything. I'm more concerned about running at 2400rpm forever and doing only 6.5 knots. That seems to make no sense to me. With almost double the hp as the original engine I should be able to run along at 9 knots wide open. The hull is a semi planing. Very little deadrise. I still haven't gotten the boat back home. Winter just isn't giving up this year and I don't want to cross an angry Lake Erie with a boat that probably needs more attention than I am even aware of yet. Thanks for your input, Ted

TT

IMHO, you are indeed correct, running at 2400 RPM is something to be concerned about. The sweet spot for your engines is 1600 to 1800 RPM, this is the design range for gensets, farm equipment and pumps. Harmonics are balanced best in this range by design as well as noise being less. Bottom line, reprop to get where most of the rest of us are.

 

[Nomad Willy]

If your mechanicals are dependable I'd run it home at 1800rpm .... whatever speed that is.

Why your boat is going so slow can be sorted out later. Sounds like the boat is designed for higher speeds and has a hull somewhere between an Island Gypsy and a Californian. If the boat is excessively heavy or the gear ratio is Way different .. or both it will eventially be obvious what's going on. Sounds like it's got a 3-1 gear to me.