Propeller sizing for auxiliary

[benri]

Does anybody have an idea what propeller size I should use on an auxiliary to get the maximum hull speed out of it? The outboard is a Yamaha Enduro 8HP. I was told that this particular model is very torquey and it should push my 19ft boat without any problems if the propeller is sized correctly.

[sirena]

hi ,i have a suzuki 8 2 stroke aged 2 decades for sure and push my 19ft cuddy cabin very well i must say.without any modifications in both propellor nor the calbulators.

Should you need any info do not hesitate mate

regards

kevin
is sirena

[blueskip]

What you are looking to get is the maximum revs the engine is capable of at WOT, any prop that stops you getting maximum revs is either too big in diameter, or has too much pitch.
My engine is a Yamaha 55HP 2 stroke & in order to get the 5500rpm which is the recommended WOT speed I had to reduce the prop down to 11.3/4" diameter X 13" pitch, from what was on there when I bought it, the dealer took the original in part exchange for the one I needed.
So your first task is to find out from the manual what the recommended WOT speed is (probably about 5000 -5500 rpm)

[skip]

Bryan, the enduro series i think is similar to the Mercury Bigfoot etc basically a stronger lower unit (gears etc) possibly a different gear ratio.

starting point is to let us know what your current diameter and pitch is, and what speed you are getting. Also do you notice that there doesn't seem to be a change in rpm between say 2/3 throttle and full, or 1/2 and full etc.

I played around abit with my 9.8 4 stroke as I was getting nothing past 1/2 throttle and now I'm good up to around 3/4 maybe a tad less. I didn't want to sacrifice too much pitch just to get the max rpm's but then again that is what is recommended.

if you want, we could go halves on a tiny tach - http://www.tinytach.com/tinytach/gasoline.php

hopfully we both need the same unit, if not maybe we can share on shipping as the units themselves are not very expensive.

Then we can measure rpms under different loads etc and you'll know where you need to go prop wise.

[benri]

i did some research and there seems to be this high thrust, or as yamaha call it "dual thrust" propeller which seems to be specifically made for small engines used as auxiliaries on larger boats.

[Destination Sea]

My aux tohatsu 9.8hp 2s (Recom. WOT 5000to6000rpm) is equipped with the standard prop 8.5x7.5 inch 216x190mm . It pushes the 17 ft (weight about 1200kgs in all) quite good. about 5.5kts in F3 wind.
The tohatsu 9.8 2s manual availabe props (a range of 5 props are availabe are as follows Starting from light boats to heavy boats
1. 8.9x 10inch  226x255mm
2. 8.9x 8.3inch 226x211mm  (standard short shaft 8Hp and 9.8 Hp)
3. 8.5x 7.5inch 216x190mm   Standard on  long and extra long shaft 9.8hp
4. 8.9x 7.0inch 226x 178mm  (standard on 8hp long and extra long
5. 8.5x 6.5inch 216x165mm

.Hope this helps and gives you and idea.