Engine RPMs vs. Hull Speed

Maine Sail · Apr 12, 2010

garylinger said:
Is there an easy way to figure out what pitch I currently have? When the boat was out of the water I cleaned the prop very well and didn't see anything other than what looked like "H P M" stamped into it. I really don't want to spend the money to buy and then install (pulling out of the water) a new prop if the prop I have is the same pitch as a replacement!

It should be stamped on the hub of the prop between the blade roots. Why would a new prop be the same pitch as one this is off by 700 RPM? If you use a reputable prop shop to size your prop it absolutely should not be the same pitch as the one you have or they failed rather miserably in their calculations. Most calculators such as Michigan Wheel's calc get it within an inch but occasionally it can be off by two. My CS prop was off by 1" of pitch plus a little cupping.

ronbo · Apr 13, 2010

A lot of opinions here.

I have a Westerbeke 82 and what MaineSail said is correct about Westerbeke's advice that the engine should be propped to attain maximum RPM.
If that's so, why does Westerbeke spec their 61 hp generators at 1800 RPM max continuously...day-in-day-out? Seems a contradiction, no?

Ronbo

Dan Johnson · Apr 13, 2010

Druid...

on your 2 vs 3 blade point, on the last boat I raced on, we marked the shaft to show when the 2 blade was verticle (hence "hidden" by the keel) and locked the trans in gear so it stayed that way when racing.

Stu Jackson · Apr 13, 2010

ronbo said:
A lot of opinions here.

If that's so, why does Westerbeke spec their 61 hp generators at 1800 RPM max continuously...day-in-day-out? Seems a contradiction, no?

And a lot of very good engineering recommendations.

The reason is that an engine designed to run a generator has a different duty than one used for propulsion.

Roger Long · Apr 13, 2010

ronbo said:
If that's so, why does Westerbeke spec their 61 hp generators at 1800 RPM max continuously...day-in-day-out? Seems a contradiction, no?

No, the injectors may be different. Even if not, it's the torque vs engine speed relationship that is important, not the actual RPM. Too big a prop upsets that relationship all the way down the power curve. The generator will also have a breaker to prevent it from putting too much load on the engine at 1800 RPM.

You could have your prop selected to match the engine at a lower RPM but you need to have the fuel system set up for the de-rated engine. That's very different than having the prop holding back an engine set up to deliver more horsepower.

MagothyBoy · Apr 13, 2010

When you have too much pitch, the engine cannot turn to meet its rated maximum rpm. Think about trying to run a standard tranny auto up hill in say forth gear. If the hill is just the right slope, you will be able to run without stalling but will not be able to gain speed (engine rpm) no matter how much fuel you supply. This is analgous to having too much pitch on the prop (being over propped). This is really not good for the engine.

I have my boat set so I can actually run the engine beyond is rated max rpm (under propped). This is analguos to running you car in say second gear. If you want to, you could floor it and run the engine beyond redline. I like having the boat a bit under propped so that when I am running into a really snotty head sea, where the waves will tend to try and push the boat backwards against the prop, the engine will not strain against these peak loads. just runs as smooth as silk. I give up a knot of top end speed under flat conditions but I like the trade off for the snotty side. Just my preference.

Hope this helps.

Kevin

druid · Apr 13, 2010

BobM said:
6.5 knots x 1202 = 7813 / 3600 rpm = 2.17 pitch?

Let's see 5280 ft/mile x 12inch/ft / 60 min/hr = 1056 * 1.15 mph/knot = 1214 so it seems 1202 is ~correct.

What's wrong here?

It's the answer you get if you don't take slip into account. Walt said slip didn't matter.

Oh, and you need to use SHAFT rpm, not engine rpm. That would 3600/(whatever your transmission ratio is).

druid

walt · Apr 14, 2010

It seems that what is recommended here is to size the prop to get the manufactures recommended RPM.

I think we would agree that at a very low hull speed, it takes little thrust to move the boat and when you are up against hull speed, it takes a lot of thrust. So at low speeds, the prop travel is going to be close to the boat speed and near hull speed, the prop travel is going to be much higher than boat speed. I.e., the prop slip is much higher near hull speed than at low speeds. As mentioned before, prop slip is more about hull efficiency.

In the case where you are trying to get the manufactures rated RPM, you almost for sure are pushing hull speed - and the drag of the hull is going to be very non-linear, i.e., small changes in speed result in big changes in hull drag. Also, the drag of a hull in general likely goes up with the square of hull speed.

So Druid, what is the prop slip to use in the equations when the engine is near max RPM (as this is going to be used to size the prop). Is it 10% or is it 50% or maybe even 100%? As mentioned, hull drag and hence prop slip are very nonlinear at hull speed and very small changes in hull speed result in big changes in prop slip. I have no idea what number to use in the equation? If you know, please explain..

Maine Sail · Apr 14, 2010

For those of you interested in the more detailed aspects of props I would recommend "The Propeller Handbook" by Dave Gerr. It answers every question raised thus far.

http://www.amazon.com/Propeller-Handbook-Installing-Understanding-Propellers/dp/087742988X

Ross · Apr 14, 2010

Mainesail has given the best advise available. Hp requirements and propellor slip and speed combine to form a very complex curve that is nearly flat at low speed (1-2 knots) and nearly vertical above hull speed.

walt · Apr 14, 2010

In answer to my question about what prop slip is.. this could be calculated based measured data.

For example, if someone had tuned the prop for the manufactures recommended PEAK engine RPM, and knew what the gear ratio was (to know the prop exact rotation), knows the pitch of the prop and also measured the exact boat speed for this RPM, you could calculate what the prop slip was.

It would be sort of interesting to know what this number actually is. But the measured slip would only apply to another boat configuration which was very similar in every aspect.

sailingdog · Apr 14, 2010

That's a great book. IIRC, Dave Gerr mentions that there are far too many variables in calculating what prop a boat needs to do anything more precise than get in the rough ballpark, and that only real world testing will actually nail down what prop is perfect for a given situation.

Maine Sail said:
For those of you interested in the more detailed aspects of props I would recommend "The Propeller Handbook" by Dave Gerr. It answers every question raised thus far.

http://www.amazon.com/Propeller-Handbook-Installing-Understanding-Propellers/dp/087742988X

Ross · Apr 14, 2010

Walt, in theory one inch of pitch at 1000 rpm will produce one knot with no slip. You can work from there to determine slip as a ratio of actual speed to theoritical speed.
Example: I have a 13x13 three blade prop and a 3:1 reduction gear. At 3000 engine rpm I get 1000 shaft rpm and a theoritical 13 knots but in realiity I get 5.5 knots. Therefore my efficiency is 5.5/13 or 58 percent slip and the efficiency is about 42 percent.

walt · Apr 14, 2010

58% slip.. interesting..

This possibly should be a separate topic but its an issue I'm dealing with right now with a new outboard and related to this topic.

In this thread, the prop is sized to give the rated RPM at full throttle.

The Honda BF8 that was on my old Mac 26S for 20 years had an 8.5 pitch prop and it worked fine. The engine still runs strong and when the carb is clean starts fairly easily but I have had starting problems in the past at high elevations. I replaced the Honda mostly because I wanted electric start.

Using the specs on the new Honda 8 HP, it says the RPM should be about 5000 and the gear ratio is 2.33.

If I plug this into an equation with zero prop slip, it says I should have been going 17.27 mph. More than likely this outboard never reached 5000 RPM and also would have had some slip (real peak speed likely just under 7 mph).

Regardless, according to how props pitch is discussed here, I had the WRONG prop - too high of pitch. However, I never really had any issue with this prop, it moved the old Mac along just fine and I used this prop at Lake Powell doing well over 100 mile motoring trips. And the motor seems to be still in good mechanical shape.

So how important is it really to set the prop to reach full RPM at full throttle? My data point says it wasn't important with a gas outboard. I'm thinking about using too high of pitch prop again because I like the lower RPM, lower noise, likely better gas mileage of having the prop pitch too high. I really don't need the peak power of the outboard (might of course imply I picked one with too much HP, the new one is a Nissan 9.8 HP - necessary to get two cylinders and electric start).

Ross · Apr 14, 2010

When you read the "propelllor Book" you will find terms like "disk area ratio" Take the area of the diameter of the prop and compare it to the area of the blades and you get the disk area ratio. The entire subject is very complex. I only carry 10 hp diesel but because of my reduction gear and rather large prop I develope low speed thrust rather than the higher speeds often expected from engines and boats that may get up on a plane.

druid · Apr 17, 2010

Just to illustrate the Voodoo involved: The most accurate prop calc program I've found (propcalc.xls) says I need 14x9 (14" dia, 9" pitch) three-blade for my Crown 28. I had on a Campbell Sailor 14x8, and I was only getting about 2600 rpm WOT (needed at least 3400). I just put on a 13x7.5, and I'm now getting 3300 rpm (close enough!), and putting up about a 3ft wake at WOT. Engine is WAY smoother and nicer (the coffeepot doesn't rattle on the stove any more!).

So: the calculations get you to a starting point, then your speed and rpm at WOT will give you an idea of where to go - I got lucky and got it right after only 2 trys!

druid

Maine Sail · Apr 17, 2010

druid said:
Just to illustrate the Voodoo involved: The most accurate prop calc program I've found (propcalc.xls) says I need 14x9 (14" dia, 9" pitch) three-blade for my Crown 28. I had on a Campbell Sailor 14x8, and I was only getting about 2600 rpm WOT (needed at least 3400). I just put on a 13x7.5, and I'm now getting 3300 rpm (close enough!), and putting up about a 3ft wake at WOT. Engine is WAY smoother and nicer (the coffeepot doesn't rattle on the stove any more!).

So: the calculations get you to a starting point, then your speed and rpm at WOT will give you an idea of where to go - I got lucky and got it right after only 2 trys!

druid

Druid,

You can't use a normal prop calc for the Campbell. Norm, if I am remembering our conversation correctly, uses the Michigan Calc with added calculations / algorithm for the CS prop. The CS is very, very efficient and even Norm often over sizes it. There are other power boat props that also require more calculations than a DIY can find online..

druid · Apr 17, 2010

Maine Sail said:
Druid,

You can't use a normal prop calc for the Campbell. Norm, if I am remembering our conversation correctly, uses the Michigan Calc with added calculations / algorithm for the CS prop. The CS is very, very efficient and even Norm often over sizes it. There are other power boat props that also require more calculations than a DIY can find online..

Yes, the "rule of thumb" for the CS prop is to take two inches off the normal calculation... sorta. (the reason I had a 14x8 was that the guy I got the engine from assured me the trans was 2.63:1 - turned out to be 2.36:1) That still doesn't work though: the propcalc program predicts 13x11 - take off 2 and that would be 13x9: way too steep, apparently.

I've had many "discussions" with Norm about their pitch numbers. If their 8-pitch prop pushes the same amount of water as a "normal" 10-p, it should be called a 10-p. They call it a feature, I call it incorrect sizing. But apparently they go by the angle of the blade, not how much water it pushes. They're great props, but hard to size correctly!

But the point was: don't put too much faith in theory, calculations or "rules of thumb".

druid

Maine Sail · Apr 17, 2010

druid said:
But the point was: don't put too much faith in theory, calculations or "rules of thumb".
druid

Which is why you'll always hear me say HAVE A PRO SIZE IT and get them to guarantee their sizing recommendations... I asked Norm to do this and he stuck to his word and re-did my prop at no charge to me.

Oh and Norm is right pitch is always measured in degrees/angle regardless of how efficient the prop is.

P.S. Apparently your PO was dyslexic..