CONTINUOUSWAVE

Hello all. I have an 1985 Striper 17 that came with an 2006 Honda BF90 and 13 x 15 four-blade propeller. The outboard engine will not accelerate to much over 4,000-RPM at a boat speed of 32-MPH. The engine runs well and [the throttle is advancing to] WOT.

To anyone that has a similar set-up: what propeller are you using?

Thanks.

From your owner's manual for your outboard engine, tell us the manufacturer's recommended full-throttle engine speed range. As your engine performs now, only accelerating to 4,000-RPM seems low for full-throttle.

Manual says 5000-6000 RPM, power curve peaks at 5500 rpm, so would like to be close to that.

[Deleted several remarks which seemed off the topic of performance--jimh.]

Is 32 MPH [top speed] excessively low for [a Montauk with a Honda 90]?

My tach maybe off slightly, but not sure if I should be expecting much more speed. Thanks.

A classic Montauk with 90-HP should have top speed of 40-MPH or higher.

Compare at

http://continuouswave.com/whaler/refere ... power.html

If a Honda 90 engine that should accelerate to 6,000-RPM is only reaching 4,000-RPM under load at full throttle, there are two possible causes or general situations that could cause this:

-- the engine is in bad tune or has a defect, and it is only making about 50-HP, or

-- the engine is fine, and the propeller load is excessive and the engine is bogged down under the excessive load.

I don't have a clear understanding of the dimensions of the propeller in use now. What is the propeller pitch? What is the Honda 90 engine's gear ratio?

Without accurate engine speed and boat speed readings, interpretation of performance results will be misleading. An accurate measurement of engine RPM is critical for proper analysis. If the accuracy of the tachometer is suspect, then so is the data it provides.

Boat speed measurement by GNSS speed over ground (SOG) data must be interpreted to remove any effect of current in the water to allow reasonable calculation of speed through the water. GNSS data for SOG will be accurate to about 0.4-MPH and should be averaged over several readings to eliminate flutter in the calculations. Note that most GNSS receivers do not actually measure speed; they deduce speed from time and distance.

Thanks for your reply Jim, I'm fairly confident in the accuracy of the speed (zero current in my lake, multiple devices and checked more then a few different times) so I guess I will have to calibrate the tachometer before I go further. The prop is 13" in diameter and has a pitch of 15. Gear ratio is 2.33:1. Thanks!

Some data is wrong. Using your data values

RPM = 4000
Ratio = 2.33
Pitch = 15
MPH = 32

The calculated SLIP is -31. That is an unreasonable result. SLIP should be about 10 percent, and not a negative number.

If we set SLIP to be 12 and calculate RPM we get 5,965. I interpret this as follows: the tachometer is way off and the engine speed is at rated maximum and possibly being limited by a rev-limiter.

My advice to you: check the tachometer setting; make sure the tachometer CAL is set for the proper engine tachometer signal.

Is the tachometer an OEM Honda four-cycle engine tachometer? If not, the readings are likely way, way off.

To get to 40-MPH at 6,000-RPM with 2.33 gear ratio and a more normal 10 SLIP, the propeller pitch would be about 18-pitch per the calculator.

Wow Jim, that's some very technical calculations you have done here. I will look into these things and let you know what I find. It is a Honda Tach tho, so we'll have to see I guess. First thing will be to get the tach sorted out. Thanks for all your help, much appreciated.

The Honda OEM tachometer is encouraging, but check its wiring and setting to make sure it is properly adjusted. You should also be able to judge the engine speed by ear, as the tone of an engine running 4,000-RPM is quite different from 6,000-RPM.

Also double check the propeller pitch markings.

The WOT expected speed for a classic Montauk with a 90hp motor, running solo and very light is 41-44 mph.
The prop you have installed is too small in pitch. (think of driving on a highway in 3rd gear instead of 5th.)

Here is one good recommendation and performance report:

Andy, aka Fishcop a well respected member here reports:

1986 Montauk with a 2007 Honda BF90D 20" shaft, mounted all the way down. Solas 17p Aluminum prop. Turns 6000 rpm and 40mph with a full load.

He posted additional detail further in the thread:

Boat has full 24gal fuel tank, two persons, aluminum T-top, two group 24 batteries, anchor, fishing tackle and 5gal live bait well. Motor is mounted all the way down

cf: http://continuouswave.com/ubb/Forum4/HTML/005629.html

While Andy's engine is mounted low, the motor should be 2 holes up (the top bolts in the 3rd hole down from the top).

For a good stainless prop, the Honda Solas in 17" is recommended.

bradwolf wrote:Wow Jim, that's some very technical calculations you have done here.

No, the calculations are very simple and make very general assumptions about how a propeller works. There is really nothing technical about them. The boat speed that occurs is a result of several factors,

--engine speed, that is, the rotational speed of the power head's output shaft;
--gear ratio, which tends to slow down the rotation speed of the engine output shaft to a lower speed to turn the propeller shaft;
--propeller pitch, which is generally a measure of how much distance the propeller would theoretically advance in one revolution as a screw, usually in inches;
--propeller slip, which is the difference between the theoretical advance or pitch, and how much the boat actually did advance in one revolution, expressed as a percentage of the theoretical advance or pitch.

And the equation describing the relationship is just a simple linear relationship. Solving for one of the factors when the other four are known is not "technical" but rather just Algebra in the form k(A x B x C x D) = E. (Here k is a conversion factor to bring all the units into proper relationship). For a bit more detail, see my article on propellers at

http://continuouswave.com/whaler/reference/prop1.html

Re HONDA OEM tachometers: I do not have any first-hand knowledge about them. I assume that the tachometer has some sort of calibration control which must be set to a particular setting to match the particular engine with which it will be used. Consult the installation instructions for the HONDA tachometer to discover the proper settings for the particular engine you have.