My boat is slow?

[sam]

Weight of driver?

[hartland boater]

Wow what w wealth of information! sounds like I may lift the motor another inch. and check on getting the right prop. I drove it today a was getting 36 mph when i shifted my weight to the back of the boat. I have a sirometer to check RPMs tomorow. I think my expectation were a little high for a fishing motor. I got $550.00 into the whole setup. its cool to operate. kind of was looking for more speed and get spooked a bit. the race engines are quite expensive. you get what you pay for. thanks for all the advice.

[ZUL8TR]

Theoretical boat speed formula

Your speed is not unreasonable to expect from the stock engine, gearcase and slightly reworked prop. Your problem is running out of rpm with the stock gearcase & prop. I put an old stock 18 omc on a c runabout and it ran 34 mph but was pegged at 6300 rpm....the omc rpm was limited by reed float, not power or prop or weight. Tells me you need a significantly higher pitched prop, stainless, and better gear ratio, if you want to run in the 40's. Note that the mini grand prix boats running 25 hp stock foot engines on small tunnel hulls with 15 or 17 pitch props are running in the mid 40's. I wouldn't expect your 15 to do that well in the stock form. Suggest you look into a gear foot if you really want to go that fast. Here's a nomograph that works pretty well with stock engines/props (not as accurate with racing hulls/engines). 10 percent slip works good for heavy boats and stock gearfeet, 5 percent for light boats and stock gearfeet, and not much of anything for racing hulls/gearfoots. Just a rough estimate for judging if you are in the ballpark or not.

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For those who want to use a calculator in lieu of the nice nomograph:

Theoretical speed(mph) = Pitch(inch) x Engine revs (RPM)/(1056 x gear ratio)

gear ratio = number of drive gear teeth/number of prop gear teeth

Your example:
Pitch = 10"
RPM = 4500
Gear ratio = 21/12 = 1.75

Theoretical speed = 10 x 4500/(1056 x 1.75) = 24.4 mph

The 10% slip for the conventional lower unit is reasonable.

The charts and formulas based on the assumption that propellor behaves the same as a screw advancing in a solid medium without slip. Surfacing propellors are much more complecated than this assumption. But in my own hydro testing the theoretical speed calculation works quite well and I find slip is in the 3% range. Can't have zero slip in reality.

Also note high speed propellors (especially surfacing piercing designs) have variable pitch all over the blades. Pitch used in the nomograph and formula is the average pitch for the entire prop not just at one or a few points and this is not that easy to determine unless you prepare a pitch contour map of each blade with some of the latest laser computer tech and calculate an average for all the blades.

[crazymanme2]

This will make it easy for you.Just punch in your numbers & it will do the rest.

http://continuouswave.com/cgi-bin/propcalc.pl

[johnsonm50]

------------------------------------------------------------------------

For those who want to use a calculator in lieu of the nice nomograph:

Theoretical speed(mph) = Pitch(inch) x Engine revs (RPM)/(1056 x gear ratio)

gear ratio = number of drive gear teeth/number of prop gear teeth

Your example:
Pitch = 10"
RPM = 4500
Gear ratio = 21/12 = 1.75

Theoretical speed = 10 x 4500/(1056 x 1.75) = 24.4 mph

The 10% slip for the conventional lower unit is reasonable.

The charts and formulas based on the assumption that propellor behaves the same as a screw advancing in a solid medium without slip. Surfacing propellors are much more complecated than this assumption. But in my own hydro testing the theoretical speed calculation works quite well and I find slip is in the 3% range. Can't have zero slip in reality.

Also note high speed propellors (especially surfacing piercing designs) have variable pitch all over the blades. Pitch used in the nomograph and formula is the average pitch for the entire prop not just at one or a few points and this is not that easy to determine unless you prepare a pitch contour map of each blade with some of the latest laser computer tech and calculate an average for all the blades.

I use the calculator, same way. With known Rs & speed you can subtract real speed from potential speed to judge the efficiency of the prop. Im using surface props & they vari from 90+% to as low as 70-% [2 bladers].

[ZUL8TR]

Boat speed

This will make it easy for you.Just punch in your numbers & it will do the rest.

http://continuouswave.com/cgi-bin/propcalc.pl

Good find. Based on the same formula without slip

Thanks

[ZUL8TR]

Boat speed

I use the calculator, same way. With known Rs & speed you can subtract real speed from potential speed to judge the efficiency of the prop. Im using surface props & they vari from 90+% to as low as 70-% [2 bladers].

Because average pitch is the most difficult measurement to obtain and the other items of RPM and gear ratio are very easy to measure, pitch accuracy has the greatest effect on the calculation. Therefore the method for pitch determination is significant. What method are you using for the overall average pitch of the props you test? Different methods here will greatly influence the result for theoretical speed. Some use pitch as measured at the 70% radius using the average pitch from the leading edge to the trailing edge (not including cup)at that radius. Others include cup. And there are other ways to measure average pitch some more accurate than others. To get around this problem I use a pitch measurement method that is the same for all props this way there is consistency and the results are comparable on a relative basis even though the absolute theoretical speed is not known.

[johnsonm50]

Thanks Z,, For now Im goin by what pitch was stated or marked, dont have a guage yet. Theres a gray area there.. Pitch accuracy then guessing how much loss is resistance &/or slip. I look at it anyway tho.

[johnsonm50]

Wow what w wealth of information! sounds like I may lift the motor another inch. and check on getting the right prop. I drove it today a was getting 36 mph when i shifted my weight to the back of the boat. I have a sirometer to check RPMs tomorow. I think my expectation were a little high for a fishing motor. I got $550.00 into the whole setup. its cool to operate. kind of was looking for more speed and get spooked a bit. the race engines are quite expensive. you get what you pay for. thanks for all the advice.

If you keep at it theres almost always room for improvement.. wanting my stuff to go faster was the best incentive to lose 15 lbs I ever had When you make changes I would keep notes & not make too many at once so youll be sure whats working. If your motor has a stream to indicate that its cooling you could run it thru a small hose up front to where you can see as you go, an idea I heard. [point it away] Then raising your motor wont do any worse than just not go. As Ron said plugging the top holes will prevent its sucking air. With any prop you run dont give up till you run it high, [except aluminum]- just never going to perform like SS.
I worked on a shift foot from an older 25 OMC -non thru hub. Over several years I got it to go from 36 to 46 on a runabout then 52 on a C hydro. By then it was coned, streamlined, clearanced for prop space, the clutch dog was pinned tight into forward gear, the shift links gone, holes plugged & reverse gear was ground toothless acting only as a spacer. Now a pal has it still doin near 50. I run rat motors with adapted feet lately. Heres my newest backyard engineered [un-cosmo-ed] 22ci 25 hp

As you get faster youll find it to be more serious FUN! Dont forget safety. Also consider racing.. very fun way to go fast as heck & be well protected as you do. I just started, its cool. Going to see is too. Mike