What the heck- I have this propeller problem

[bntii]

I have re-powered a 20,000 lb sailboat with a 35 hp Kubota diesel rated at 3000 rpm, and have launched the boat with the propeller recommended by the prop shop.
The boat has 28.3' load waterline which yields by my reckoning a 7.2 knot "hull speed"
I have taken the boat out and see the following:

Engine runs up to no load rpm as from manufacture: 3200 rpm

Well powered- she runs up nicely seems the have plenty of umph.

The boat is seeing 6.2 knots at 2000 rpm

At 2500 rpm the engine is maxed out and the speed is 7.0 knots and the temp is a bit high at 210 F.

The question I have has to do with understanding what I am seeing on the water.
Is the rpm not reaching the 3000 rated for the engine due to the boat being over pitched? or.... it the fact that the boat has reached near the hull speed coming in to play? That is is the power required to move the hull faster limiting any increase in speed through the water and hence the low observed max rpms?

I have from one net source that the vessel should be tied to the dock and the engine run up in gear forward to test the max rpm's reached with the factor of hull speed removed...?
I suppose this line of reasoning indicates that the max rpms would increase if head seas/wind limited the speed of the vessel though the water. I have also read that one should pitch the prop to reach hull speed lower in the range so as to leave some reserve for pushing to weather.
I guess this whole question comes down to can the correct pitch on the propeller be expected to push this heavy boat to more than 'hull speed'??

Thanks all

[CTMD]

Yes you have to much pitch/dia/area. Your prop shop should be willing to swap you for another better suited prop and if you give them the above info then they'll be able to make a more accurate "guess". A correctly pitched prop may give you a slight increase in speed but it will definately give your engine a longer happier life.

[masalai]

CTMD, Is it my ignorance and lack of this sort of knowledge, I hope to learn here too...
I would have thought maybe 6.2knots may be the mark and the efficiency would be better served to add a bit more pitch so the engine would be happy at MAX TORQUE revs and run a bit less than that to reduce the load as at "hull speed" she would be climbing already.
Look at a video you your boat in still water and is there a "full cycle" in the wave form between the water at the bow & the stern?
No point in trying to climb out of the water (on to the plane), unless you are adept at pushing S**t up hill with a pointy stick...

[CTMD]

Its important to remember when reading the following answer that I'm a Naval Architect, not an engine expert.

My experience is this:

1. If a boat can't get to its RPM the engine supplier will usually void the warranty as the engine is being "overloaded"

2. As i mentioned in my last post reducing the pitch will allow the engine to reach full revs but will have little effect on available speed (sometimes a very slight increase)

Your engine needs to be able to run within itself across the rev range and for max efficiency get access to the full range In order to do this it needs to reach full revs at its maximum speed.

To use an analogy think of a three speed bike. When going up a slight hill if you work really hard you can use top gear get good speed but will quickly tire. Change down a gear you'll get similar speed but by increasing your RPM you taking the load off your legs you'll be able to do it for longer. Change down a gear again and you'll pedal like ****, go nowhere and get tired quickly. You want to pitch your prop so its in the middle gear region.

[masalai]

Thanks Chris

[TollyWally]

Your analogy makes a crisp mental picture. Just the sort of thing I need to grasp some of the basics. More please!

[FAST FRED]

NO one usually wants to cruise at "hull speed" because by backing off a bit the fuel consumption will frequently be cut in HALF , for about a 1/2K.

Sounds like your instalation has what is known as a cruising prop, intentionally larger in diameter and pitch than would allow eng mfg max rpm.

This is GREAT !!! , if you can accept the slightly lower speeds , in exchange for longer engine life and less noise in the vessel.

At 6.5K a boat like yours would only need about 15hp, about 3/4- 7/8gph.

I would run full throttle and pull back 300rpm and see how the engine likes that speed , load and measure the fuel use.

"I have from one net source that the vessel should be tied to the dock and the engine run up in gear forward to test the max rpm's reached with the factor of hull speed removed...?"

This WILL NOT WORK! as a proper prop is designed to be moving forward (6k would be my guess for your hull) not tied to a dock.

If you had a 100ft tug this might be somehow usefull, but for a sailing aux the proper prop not advancing will cause the engine to overload , and blow black smoke. MUCH UNGOOD!

See if you get about 6.5 underway with the reduced (300) RPM , and if you do PAINT a red line at that rpm on the engine tach and have fun!

Just NEVER operate at full bore, as the proper Cruising prop will overload the engine .

FF

[TollyWally]

I am quite interested in the cruising prop concept. Coould you expand on that a bit?

[bntii]

Ok- thanks all.

Trying to get my head around what is apparently a simple setup issue.

CTMD- your bike analogy sets it very clearly though I am still working over the particular aspects of the relationship of pitch to speed through the water.

You state:
"Your engine needs to be able to run within itself across the rev range and for max efficiency get access to the full range In order to do this it needs to reach full revs at its maximum speed."

How is this full range setup reconciled with the ideal that a fixed blade prop can be (must be?) optimized for a particular speed?

I am getting the feeling that the setup can be designed so that the propeller pitch/diameter can be selected for max speed or for max power.
In case A the vessel would achieve a higher speed through the water at full rated rmps.
In case B the vessel would achieve a lower speed at full rpms but would have a higher resistance to damping forces- wind, headseas, etc.

Am I just making this up?

I am guessing but think the power side would be a larger prop and flatter while the smaller greater pitch prop would favor speed...?? Is it possible that in either case if full rated rpms would be reached but that the 'character' of the vessels power curve would vary?

Fast Fred - is it correct to run in the middle range at the highest point on the engines torque curve and lower rpms?

My boat is a cruiser and I am most interested in punching poor conditions if there is in fact some latitude available in the adjustment of this prop setup.

Edit: And another thing! ... I have heard it said "set the propeller so that hull speed is reached at 1/2 revs in calm water so that there is 'reserve' for headsea conditions" I also hear that the engine should be run at 80% revs cruising for longevity of engine.... Are these folks simply running at lower than 50% revs or are they barreling along at over hull speed in calm conditions...??

Tag line: Not as dumb as I seem.... just another bloke with a boat

Beanz:

http://www.dieselenginemotor.com/kub...1349-v1505.jpg

[Meanz Beanz]

I'd be going for max speed at max torque and forgetting the top of the revs. The top speed sounds fine, if the engine is a peak efficiency at the top speed then .... then again I am just an ape What does the torque curve for the engine look like? Sounds like the prop might not be far off at 2500 RPM for top click. and yes I reckon you are probably seeing the max possible out of the hull give or take a smidge.

[SamSam]

I have from one net source that the vessel should be tied to the dock and the engine run up in gear forward to test the max rpm's reached with the factor of hull speed removed...?

The factor of hull speed, the hulls resistance to going faster, would seem to be guaranteed and maybe even multiplied by some sort of factor. Maybe instead of the prop being more or less fed 6-7 knots of water and then using all the engine's power to reach it's full rpm, it now would have to pull 6-7 knots of water and then use what power is leftover to reach it's full rpm? That could be just abstract blathering, but it sure seems that tied to the dock, the hull speed factor would be guaranteed.

BTW, how come there aren't any 2 or 3 speed drives for boats? It works for cars, why not boats? Or planes?

[masalai]

As a displacement hull with a small engine, she will only do what she will do any more - think in terms of a hundred horsepower and more. Look at how your boat behaves in calm waters, once the bow wave creates a nice sine wave form to the stern in calm waters - so you can see it happen. THAT IS YOUR LIMIT... She won't go faster so back off and enjoy a fuel efficient ride...

[Meanz Beanz]

OK. if I am reading that right you will be hitting max torque toward 2200 rpm. I would guess that would be around the peak fuel efficiency. If you can get to the 7 knots around peak fuel efficiency & torque that sounds fine to me, in fact having to rev the engine a little harder is OK in my book as yacht diesels need to work. More small yacht engine suffer from glazing up from low loadings than ever get over loaded. I'd talk to the engine company but it sounds like you are not to far out of the ball park to me, I'm not convinced of the need to run to peak revs at peak speed... but hey if they tell you different who am I to argue? Lets us know if you get the juice from Kubota... nice engine choice by the way

PS... I am no expert!

[Meanz Beanz]

Anyway I reckon that prop shop have "dropped you on the green", you might be able to do better but how much? and will it be worth the agro?

[bntii]

thanks all- prop guy says to pull an inch off so in the water I go.

boats are a project- cert way overdue on my tanks- off for a hydro and dropped off for a fill at the dive shop

was out of beer but thats a quick fix.

ya- that Kubota is a pusher no problem there and will be fine with a bit taken off the prop