Prop Advice

[Kiwijohn]

Just some advice please - I have recently repowered my 4.8m Nesscraft (Cuddy) with a Tohatsu 70hp and now I am trying to get the prop right.

Present figures with two adults and fishing gear –

6000RPM (May be able to push it slightly higher but as I was over RPM range didn’t want to push it)
30 Knots
Standard Tohatsu prop – pitch 15, Dia 13.8

I am presently using the boat for fishing in the bay but am looking at some diving in the future – so will be carrying a fair bit of weight.

My options so far are

Tohatsu prop – pitch 17, Dia 13.3
Mercury Black Max – pitch 17, Dia 13.25

I am new to all this and so look forward to your advice.

Thanks

John

[kizza1]

whats the max rpm of the engine. is the engine hieght right? does it cavitate on turns when trimed in, does it spray heaps when trimed out?

17 will probably drop the revs by about 200-300 rpm.

[nutgunaspu]

Hin John, I've just been through the same drama as you and my experiences may be able to help a bit.

I recently fitted a 140 johnson to my 5m half cab, and with a 15 x 17 prop it revved out to 6800 at wot!:o I fitted a permatrim foil and the revs dropped to 6100 at 36 knots. :-/ Fitted a 14.5 x19 inch prop and it dropped the revs at wot to 5600 at 36 knots. :)

It might be worth checking if the overrevving is just due to the prop or if there may be any other contributing factors, as my motor must have been sucking in air to over rev to 6800 rpm.

[Seahorse]

hey nutgunaspu,
what was original speed at 6800. 36kn?
this is very interesting. so u r now doing same speed and hopfully using less fuel.
I see on here quite often about guys trying different props.
Hey these props arnt cheap. is there anywhere where u can try these props without the cost of buying.
seems to me to be bit expensive experiment.
one always wonders if u could get any better performance out of ur motor.
i got 5m stacer runabout with 75hp 2st merc. its got vengence 18p prop. i get top speed of 31knots at 5800.
just curious if anyone else has similar.

cheers
greg


cheers

[FNQCairns]

Just some advice please - I have recently repowered my 4.8m Nesscraft (Cuddy) with a Tohatsu 70hp and now I am trying to get the prop right.

Present figures with two adults and fishing gear –

6000RPM (May be able to push it slightly higher but as I was over RPM range didn’t want to push it)
30 Knots
Standard Tohatsu prop – pitch 15, Dia 13.8

I am presently using the boat for fishing in the bay but am looking at some diving in the future – so will be carrying a fair bit of weight.

My options so far are

Tohatsu prop – pitch 17, Dia 13.3
Mercury Black Max – pitch 17, Dia 13.25

I am new to all this and so look forward to your advice.

Thanks

John

Going up in pitch will lower rpm, how much will depend on the particular hull, engine and setup.

Going down in dia will raise rpm, how much will depend on the hull, engine and setup.

Also going up in pitch will increase speed at most every rpm obtainable, some caveats apply, so more hull lift at each rpm which reduces slip in comparison to the slower prop.

Think you will be happy with either 17 in comparison, only a test will tell you how happy you will be. If you are traveling at a good clip but WOT is say 200rpm higher than manufacturers recommendation it doesn't matter (it's actually a pretty good place to be) you have absolute control over how high it revs via the throttle.

cheers fnq

[nutgunaspu]

Hi Seahorse,
from memory speed at 6800rpm was about 34 knots. I didn't spend too long at that speed as I was waiting for the motor to go bang. I bought the 12.4 x 19 on ebay for about $50, so it wasn't too expensive .

[Kiwijohn]

Thank you for all your replies - some great advice there.

Kizza - recommended max is 5850rpm

Thanks again

John

[Fishnwell]

KiwiJohn,

Been down the same track with two boats now and have no hesitation recommending The Propellor Warehouse 07 54379400. Steve or his son will talk through all the issues recommend a prop and if you are not satisfied, they will work with you or you can return the prop. Personally I have found their 4 blade props better for off shore work.... Lose a bit of top end but definately carries the boat better in the chop......

[Seahorse]

fishwell
whare are they

greg

[Outsider1]

Propeller Warehouse is the name of the Solas' outlet. They are at Warana;

http://www.solas.com.au/shopcustcontact.asp

Cheers

Dave

[jon80]

Kiwi john,
prop that works on 60-90 tohatsu is a mercury 14inch vengance I have worked on tohatsu for nearly 20 years now, have spent alot of time prop testing, vengance 14 will suit your needs , try & find someone with one.

[kizza1]

Kiwi john,
prop that works on 60-90 tohatsu is a mercury 14inch vengance I have worked on tohatsu for nearly 20 years now, have spent alot of time prop testing, vengance 14 will suit your needs , try & find someone with one.

i dont understand how you can say a 14 will work when he currently has a 15p and its 150rpm over the max rpm

i would say it either needs to go up to a 17p or drop the engine 1 hole, depending if the engine cavitates or not.

[jon80]

i dont understand how you can say a 14 will work when he currently has a 15p and its 150rpm over the max rpm

i would say it either needs to go up to a 17p or drop the engine 1 hole, depending if the engine cavitates or not.


prop dia is diff.& rake of blades are different,
also said to try and find someone with one to try
props are not props you can put 4 or 5 same pitch diff brand &
get all diff. performance,revs,accel....:)