Boat Danish from Bulgaria

[niki12]

Here, I hope you like it.

[Mark75H]

I like it. Good work!

[zul8tr]

What a cool paint job and a great transformation. Get some numbers on it.

[niki12]

Image Engine is a Russian Vihar 30 hp. But it is very old and not reliable. It develops 38 mph. Propeller speed is not.

[niki12]

With Mercury 15 hp stock makes 29mph

[niki12]

Engine Tohatsu M40C long boot, propeller 10 1/4X14 , speed which reached 45mph. Transom Height 22" .Spring will take propeller 10X19. I want to reach 60mph.Is it possible to achieve such speed propeller 10X19?

[zul8tr]

Can you provide the engine rpm for the 45mph speed you reached?

With the Toha 40hp and a 19 pitch, 1.85 gear ratio and long shaft I think you will have a tough time getting 60mph. To do that you would need the engine rpm to be about 6700 rpm allowing for 8% slip and I doubt that the 40 hp will achieve that rpm with a 19 inch pitch and the motor height you have.. Perhaps a 17 pitch.

Your transom is already pretty high but the long shaft is just too long for the transom height based on photo and raising it higher is asking alot. for the design. Get a standard short shaft and you will be on the road to improvement.

[niki12]

Engine speed I do not know how.I think his gear number is 1.92. Transom photo is slightly down. With 14P engine is not felt to fall speed, the boat is very light. Will measure the speed of the engine before making propeller. I wonder between 10X19P and 12X17P, I want to consult you for advice. How much that increase speed with two propellers, which is that possible?

[zul8tr]

I obtained the 1.85 gear ratio from the Tohatsu manufacture site for the 40 hp where did you get the 1.92? With such a light boat the larger diameter prop is generally not the way to go. With these hydros the rule is small diameter and large pitch and raise the engine up so that the prop shaft is close to the bottom. Your transom setup will have to go a long way to do that.

The difference between the 10 x 19 and the 12x 17 is a guess since the 2" reduction in pitch has a 2" increase in diameter but my guess is that not much. I would rather see a 10 x 17 compared to the 10 x 19 on your hydro.

All you can really do is test and keep notes as to what you are changing and change one thing at a time.

Here is a calculator for expected speed for pitch, rpm, gear ratio and slip factors

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

In the calculation Slip is a measure of actual vs theoretical speed. Theoretical speed has 0% slip, 100% slip is not moving. A 10% slip (90% theoretical speed) is about right for your light set up and standard fishing engine.

This calculation does not tell you if the engine can rotate the prop at the selected rpm or if the boat has excessive drag to prevent such a speed but it does provide a way to see how the different variables used relate to speed.

[niki12]

Gear ratio of the site for Tohatsu M40C 1.92. http://www.tohatsu.co.jp/en/boat/products/m40csp.html
For GPS I achieved with 14p propeller 44mph. In the calculator with 10% slip motor is 7200rpm. Even if you can not hit 60mph with a propeller which is a fast boat 17p or 19p. And most of all safely Engine