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Old 10-11-2007, 00:56   #1
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power problem

i am having a problem.
I have recently launched my 40 ft power trimaran with two, thirty hp Honda outboards and performance is well below what it should be.
With one motor at 3,000 rpm I get 6 knots with both motors at 4,500 rpm (maximum revs) I only get 8 knots.
Now before you all say 8 knots is OK for a 40 ft boat you must understand that this is a very slippery hull which weighs in at 5 tonne. At 8 knots there is NO sign of a bow wave, the water flow is very smooth and flat even the stern wave is flat.
The motors should rev out at 6,000 rpm but stop at 4.500rpm with standard props. Everyone wants to sell me a smaller pitch prop which would get my rev's up. but I do not see that increasing the speed very much.
Any suggestions?
My experience and prototypes that I developed of this design have shown that I should be getting more speed for that amount of power. My last boat a Catalina 25 full keel would do 7 knots with a 10 hp Honda outboard before a bow wave formed and hindered further speed.
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Old 10-11-2007, 02:04   #2
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Quote:
Originally Posted by beau View Post
... The motors should rev out at 6,000 rpm but stop at 4.500rpm with standard props. Everyone wants to sell me a smaller pitch prop which would get my rev's up. but I do not see that increasing the speed very much...
I presume the 6000RPM figure is the maximum at Wide Open Throttle, from your engine manual. I also presume the two engines are Counter-Rotating.

Overloading the propeller (high pitch and/or diameter) results in lower engine RPM throughout its entire RPM range for a given throttle setting.

Assuming the Prop’ Diameter is correct, the standard solution to low RPMs is to decrease Pitch.
Why don’t you believe it?


SPEED = (RPM ÷ Ratio) x (Pitch ÷ C) x [ 1 - (SLIP/100) ]

Where:
SPEED = the boat speed.
RPM = the crankcase speed in revolutions-per-minute.
RATIO = lower unit gear reduction ratio; the number of revolutions of the crankshaft to produce one revolution of the prop shaft.
PITCH = blade pitch of prop in inches.
SLIP = 0-100 index of propeller performance (a percentage) .
C = a constant to convert inches-per-minute of revolution to boat speed*:
* 1215.2 for knots (nautical-miles-per-hour)
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Old 10-11-2007, 04:45   #3
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I suggest you try props with less pitch. Some engine manufacturers won't warrant their engines unless they can reach a specified RPM at WOT. The shop you bought the motors from should be willing to let you try different pitch props till you find the right ones.

To be honest, the performance you are talking about sounds fairly reasonable for a 40 foot, 5 tonne boat with 60 hp.
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Old 10-11-2007, 13:45   #4
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For sure change the props.
You are overloading the motors.
They might be Hondas but you are taxing them quite a bit.
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Old 10-11-2007, 14:33   #5
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Yep... listen to these guys.
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Old 10-11-2007, 23:56   #6
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Before changing the props, make sure you have the motors set up properly, to include trim, toe in, depth, and overall tune. Are these new motors? Do they put out equal power or is one running stronger than the other? Are the hulls clean? First verify your install, then work on pitch. By the way, have you calculated you hull speed? 8 knots is probably pretty close. Re-propping may or may not get you much additional speed, but it should get you economy and longevity.

Brett
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Old 11-11-2007, 15:30   #7
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Thank you all for your help.
I am aware I might have needed different props because this not a planeing boat and the 30 hp Honda is designed for a small planeing boat.

I have spent many years and built a number of prototypes designing this hull which has a very sharp angle of entry and a flat bottom and wide stern. This was all based on a fast displacment/non planeing hull.

Before I purchased new props I thought it best to seek "the forum's " advise. Thank you all.

The other aspect I have noticed is that although I have no formation of a bow wave evident, even at 8 knots, I am 2 inches lower in the stern than I had hoped (extra large batteries etc etc) I could be dragging a larger stern wave than I had hoped.
So after I change the props I will move some weight forward etc and see how that works.

I appreciate the comments that 8 knots for a 40 ft boat is not bad but I really do expect that by NOT fitting into the displacement hull aspect of bow wave formation etc I can achieve a higher speed using 60 hp.
I will let you know how I go.
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Old 11-11-2007, 17:30   #8
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Aloha Beau,
How long is your waterline? If your boat is displacement then 8 knots is about max. especially if she is down a bit in the stern. If you can get your revs up to 6000 and balance a bit better you might be able to max at 8.5 or 9 but not much more than that.
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Old 12-11-2007, 01:20   #9
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This is really basic stuff but it sounds like a remedial lesson is needed...

Your engines are rated for 30 HP AT 6000 RPM. If you have them over-propped and they are only turning 4500 they are NOT putting out 30 HP, in fact not even close to it.

Put the right props on them so they actually put out 30 HP and then come back if performance isn't up to your expectations.

Something to consider, at low speeds wave-making resistence might not be your primary power issue. Skin drag might be the bigger issue, especially if you have a larger than "normal" wetted surface, which I suspect might be the case if you have a tri...

All things said, your performance looks like it is right on the "normal" curve for a boat of that size and weight. You can't fool mother nature.
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Old 12-11-2007, 02:01   #10
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See the section on Engine and Propeller Performance Curves in
The Propeller Handbook ~ By Dave Gerr
Here:
The Propeller Handbook: The Complete ... - Google Book Search
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Old 12-11-2007, 10:39   #11
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With your present props and the max rpm of 4500 the power your engines are actually giving you is around 30 hp Total or about 15 each with the right pitched prop you should be getting 11 knots top speed , at present you are killing your engines and using to much fuel
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Old 17-11-2007, 15:30   #12
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Ok, I put new prop's on from Solas, 9.9 X 9 three blade.
I got 6,000 rpm at full throttle. good.
but only a small increase in speed 8.5 knots. I can sit on 7.5 knots at 4,250 rpms.
Still no sign of a bow wave at 8.5 knots on a waterline length of 39 ft.
But i am overweight at the stern an I am dragging the stern with a fairly reasonable stern wave.
I am not happy with the present situation and will attempt a number of rectifications.
First i will move two of the four 200 amp hour batteries from the stern to as far forward as possible.
Second I will try and get rid of some excess weight.
It may be that I need more power other than two 30 hp Honda's

Gps works, autopilot works and the boat feels very good downwind. A little bit of pounding upwind because of the flared hull but not excessive and something I can live with.
Still getting used to the handling and realising that with outboards I have NO steerage without power. Even at idle I travel at 4 knots so coming in to a marina I am on and off with the throttle. But it just needs a bit of practise and the boat is far more manuvernuble than a 36 ft power cruiser I had with one motor and a rudder.
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Old 17-11-2007, 19:26   #13
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8.5 knots is pretty much hull speed for 39 feet LWL.
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Old 17-11-2007, 19:46   #14
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Hey, what happened to the rudder...no steerage??

There may be times that the sailing rig will allow you shut off the motors and cruise at a desireable speed.
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Old 17-11-2007, 19:53   #15
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Aloha Beau,
I don't think you are going to go any faster with bigger engines. You mentioned that it is a displacement boat. Doing the calculations you won't be going much faster than 8.5 - 9.0 unless you plane.
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