CONTINUOUSWAVE

I have a 1993 16SL and am re-powering it with a Yamaha F70. It will be used in Green Bay and sheltered waters of Lake Michigan. I would appreciate any suggestions for propeller selection.

BNelson wrote:...am re-powering...with a Yamaha F70.

A 996-cc four-cylinder Yamaha F70 is rated to produce 70-HP in a range of 5,300 to 6,300-RPM, and has a gear ratio of 2.33:1.

You should compute a target maximum boat speed based on the total boat weight, 70-HP, and a hull factor of 180 using the Crouch's method calculator. Then select a propeller pitch that will produce that boat speed with a SLIP of 10 and the engine speed at 6,300-RPM.

If you pass along the total weight of the boat--that is all the weight for hull, gear, engine, fuel, and passengers--we can calculate a target boat speed.

Once we have the target boat speed, we can make an initial estimate at propeller pitch.

BNelson wrote:It will be used in Green Bay and sheltered waters of Lake Michigan.

Your planned use of the boat in moderate temperatures and fresh water at an elevation of about 575-feet won't have too much influence on the propeller choice.

BNelson wrote:I would appreciate any suggestions for propeller selection.

As an example, if the total weight is 2,000-lbs, and using a Crouch's hull factor of 180, with 70-HP we'd expect a moderate planing hull to produce about 34-MPH. With 34-MPH as a target, the 63000-RPM engine speed through a 2.33:1 reduction would require a propeller pitch of about 15 with SLIP at 10. The Yamaha OEM propellers are generally quite good performers, so a starting point for propeller selection would be to get a Yamaha three-blade steel propeller of 15-pitch. Then conduct some sea trials with that propeller to see what performance is obtained.

To conduct sea trials, you will need accurate instruments to measure boat speed and engine speed. Generally a GNSS receiver will give you boat speed over ground to about 0.5-MPH accuracy. Engine tachometers vary in accuracy, but if you have an OEM Yamaha tachometer it should be good. You will then record boat speed and engine speed at various on-plane speeds, up to full-throttle.

The goal is to get the engine to reach the upper third of the maximum engine speed range when at full throttle when the boat is in its normal boat weight condition. This should give you some reserve for when the boat is heavily loaded. You will want to conduct the test in normal weather and nearly calm seas.

If the propeller under test allows the engine to reach maximum rated engine speed before full throttle, increase the propeller pitch by 2-inches and test again.

If the propeller under test does not allow the engine to reach maximum engine speed before full throttle, compare the engine speed with maximum engine speed range. You can figure that a reduction in propeller pitch of 2-inches will cause an increase in engine speed of about 400 to 500-RPM.

Adjust the propeller pitch according to the above guide, until you have found a propeller pitch that will put you into the optimum ENGINE speed range, which, again, should be in the upper third of the manufacturer's specified engine speed range for full-throttle. In your case, the F70 range of 5300 to 6300 would be reduced to 5950 to 6300 as the upper third of that range. With four-stroke-power-cycle engines the maximum horsepower is usually produced in the upper end of the specified range. For this reason, those engines should be fitted with a propeller that will let the engine accelerate to the upper portion of its maximum engine speed range with the normal boat total weight.

I suspect that a 15-pitch might be too little, so you might want to also have a 16 or 17 pitch in the same series to immediately test with, too. Propeller testing is the only way to get to the optimum propeller. Find a vendor who will let you buy, test, and return a propeller until you find the right one to buy from that vendor. An alternative is to borrow propellers from friends to test.

For links to a Crouch's method calculator and a propeller calculator see

http://continuouswave.com/forum/viewtopic.php?f=7&t=41

To further understand how propeller selection using the upper range of engine speed is important to get best performance, see

Propeller Power Curve
http://continuouswave.com/whaler/refere ... Curve.html

Yamaha Performance Series 3-Blade propeller. The ideal size 13-1/4" x 14.

Phil is right on and mount your F70 all the way up with 3 holes showing for best performance.

As the owner of an F70 (on a 15) I recommend an easy-spinning propeller with relatively small blade area to let your new motor get up in the RPM range where it makes its power. I ran a propeller with larger blades for a couple of years and was never completely happy with its performance, so after connecting with PowerTech tech support, I switched to a PowerTech SCD-3. I run a 16-pitch and am intentionally slightly under-propped, so for your larger boat I would recommend starting with a 15 and see how it goes.



The PowerTech will be MUCH less expensive than the OEM Yamaha, and PowerTech lets you trade for a different pitch in the same model for a nominal fee. The SCD-3 is available in 1-inch pitch increments, while the Yamaha is only available in 2-inch increments. I got my SCD-3 from Dan's Discount Props - they don't show all the PowerTech models on their website, but if you call they can get them. A mentioned, you can also connect directly with PowerTech for advice - they were very helpful to me.

Jim is right on - you want the WOT RPM to be at the top of the recommended range, for best performance, and trial and error is the only way to get it right. This is where PowerTech's 1-inch increments and trade-in policy will help a lot.

Another option would be to try the Turbo Quest. It is supposedly made specifically for the F70. I have not tried it, but it looks to have the same characteristics as the SCD-3, with a more rounded style on the leading edge.



But before you do anything, make sure the motor is mounted at the right height, as others have described.

Flymo

I have a 1999 Alert 17 with a Yamaha F70 ahd I run a Yamaha Performance Series 13.25 x 14 SS prop that Tom Clark recommended mounted 3 holes up just as Tom said and it is right the perfect setup on my boat.

Does anyone have a picture of the Yamaha Performance Series prop? I would be interested in how it compares, in terms of blade area, rake, and cup to the Stiletto Advantage 1 that I found to perform poorly on my boat, and the SCD-3 which was a vast improvement.

I understand that the SCD-3 is very similar to the Yamaha Painted Stainless prop, just not painted (d'oh) and available in 1" pitch increments.

Thanks Phil - I did see those photos, but because the angle is so different from the ones I posted, it is hard to compare.