swapping engines and props

[levi_tsk]

ok so i have a 15' sundance with what was a great running tohatsu m60B the gear ratio in the lower was 1.62:1
the prop was a 14p stainless cupped power tech and loaded down it would turn the max reccomended rpm of 5600 (it banged the rev limiter unloaded)

my question is this:
im getting a 60 etec to replace it and the lower unit ratio is different (2.67:1 max rpm 6000) if i take the old WOT rpm and divide by the gear ratio and multiply that number by the pitch divide by and divide by 12 i come up with about 3159 feet per minute can i use this number as a target for the new prop???

im thinking the 17p rogue 4 blade will get me back in the same range as the old prop on the M60B doing my earlier math i came up with 3189 feet per minute
does this seem right or ??

 

[Faztbullet]

Re: swapping engines and props

You should be able to go up at least one prop size ...say a 15p as the Nisshatsu according to the factory specs is close to the E-tecs(2.33 vs 2.67)...
http://www.tohatsu.co.jp/en/boat/products/m60sp.html

 

[Silvertip]

Re: swapping engines and props

The E-tec should easily spin a 17P. The universal prop for a 40/50 JohnyRude twin was a 12-3/4 x 15P or a 11-1/2 x 17P. I've done extensive testing on one of my boats with that engine. The E-tec is a twin and should do well on that size boat with a 17P.

 

[steelespike]

Re: swapping engines and props

You might go to the Etec site and check the performance tests. Typical props are 15 and 17".on boats slightly larger than yours.
I don't know anything about your Sundance so hard to suggest a prop. At 6000 17" at 12% slip would do about 31.8 mph at
a 19" at 12% 35.6.If the boat is light and fast a 19 might be possible.Very likely your tests should produce slip in the 5% range
with an improvement in speed.

 

[levi_tsk]

Re: swapping engines and props

silverspike:
its a modified v center console hull with a manual jackplate gps on my phone said about 32 mph top speed loaded im thinking that what your saying about a 19p might be better after recalculating my numbers this morning

faztbullet:
i just looked in my manual and i was wrong ...the gear ratio i found was for the m60b and came right out of the service manual say its 13:23 Or 1.76 ..... so im not sure if that page you showed is right i do know though that on the later models the gearing was lower as you have stated

have you guys had good luck getting a loanerprop on the purchase of a new engine or? - i know alot of this is speculation and ill need a baseline with the new engine but i dont wanna buy 1000's of dollars in props you know?

 

[levi_tsk]

Re: swapping engines and props

here my recalculation :

{(5600/1.76 )x 14 }/12=3712 fpm for the tohatsu

if i use that as a target #

then [(6000/2.67)x (X)]/12=3712 so solving for (X) a 20p prop would be best but id like to go to a 4 blade which would then create the need for a 19p

is the torque output for the two engines the same or is all of that irrelevent due to the lower unit difference?

 

[levi_tsk]

Re: swapping engines and props

oh and after searching the net the boat is 800lbs would you guys consider that to be light??

 

[steelespike]

Re: swapping engines and props

I would say 800 is about average. Looking over various test reports a 19 seems a little too much.
I just wonder if it can turn 6,000. While most of the boats are longer and heavier than yours they are running 15" and 17" props
and generally turning about 5800. speeds 30 to 33 mph. If your getting a new motor I would think the dealer would help you find the right prop and setup.I'd be inclined to go,with a 17 to start it should provide more hole shot and stay on plane easier.
I love 4 blades for certain applications but they generally do produce lower top speeds. I'd try 3 blades till you see what they can do.

 

[levi_tsk]

Re: swapping engines and props

ok cool anyone else wanna chime in here??