Prop advice

[Robroy]

Hi,

Been trying to re-prop the boat with 4 blade s/s

Reason better holeshot and grip in whitewater situations.

Boat: Cruisecraft 685 outsider
Motors: Twin 115 Johno/Suzuki 4 strokes
Props: 3 x 14 x 17p Alluminium


Operating range w.o.t. 5000 - 6000 rpm


1) 4 x 13 x 17p s/s

Holeshot great, up on to plane quicker
W.o.t. over 6000 rpm

2) 4 x 13 x 19p s/s

Holeshot still great, up on to plane quicker
W.o.t. over 6000 rpm (didnt let go over much
still had plenty to go on throttle levers)

3) Next to try is 4 x 13 x 21p

This combo may bring the rpm down but wont it
give a poorer hole shot?


Can having twin engines on a mono hull cause any ventilation problems being so close together?


Any thoughts or comments would be appreciated

TIA


Robroy

[Kerry]

During these trials are you fully loaded, full fuel, full everything and no point playing in the calm stuff, trial it under actual expected max type conditions.

21" is sort of starting to sound a bit high on 115's but then what's the gear ratio?

Cheers, Kerry.

[Robroy]

Hi Kerry,

Thanks for the reply gear ratio 2.59:1

As for load almost full of fuel and it is only ever my son and I in the boat.

Regards



Robroy

[SeaSaw]

Robroy, have you thought about trialing a prop with a bigger diameter rather than just uping the pitch. The extra blade area changes the performance characteristics, often increasing torgue (ability to hold a plane at low speeds in heavy conditions). Maybe worth trying as another option.

Mark

[Mr__Bean]

G'Day Robroy,

In addition to better grip in whitewater, the 4 blade definately hold better boat speed when punching through heavy chop.

Given that you have twins, I suspect that you want to be able to get home on one engine if you need to.

21's sound too high to me.

Being 4-strokes I would want to make sure the boat can get up on the plane and have enough power to get you home safely with a reasonable load on one engine (with 19's) before thinking of 21's.

- Bean

[Robroy]

Mark,


When I received the first set of props Ii asked why the diameter was 13" and not 14" the guy said this did not matter on a four blade prop.

I took his word for it as he is the prop dealer/expert.

After doing another test this morning with a mate of mine who has been boating for some twenty odd years and has carried out rescue work, we agreed there was no sign of ventilation and that it seemed like a loading problem, his comment was that he thought a 14" dia prop would provide this extra load and keep the RPM down.

In his oppinion the props performed really well as far as grip and holeshot went and going up in pitch to 19p would be a backward step.

Regards


Robroy

[SeaSaw]

Robroy,

When I was looking at new 4 stroke engines, the Suzuki's where different from all the other brands in that they run a much higher gear ratio, allowing them to turn a bigger diameter prop. In my mind this is one of the big advantages of teh suzuki's for offshore performance. Amittedly I was only looking at 225 engines, but I assume they have the same approach right through there product range. If it is true for the 115's you have, then they are actually designed to run a larger diameter prop than their competitors. I found some dealers didn't understand this and were matching the Suzi's with too small a prop, while others seemed to be in the know.

Cheers,
Mark

[Robroy]

Mark

Funny you should say that I have just spoken to the guy from Solas who sold me the props and he reckons that the gear box on the 115 4 strokes cant handle the 4 blade x 14" dia prop
how can I confirm what is the right info.


Regards


Robroy

[SeaSaw]

Robroy,

The guy from Solos knows his stuff when it comes to the right props for a particular situation. The only other place I can think of where you could get good advice (and a different view no doubt) is from Haines Marine who are the Australian Distributer for Suzuki. I spoke to Greg Haines at a boat show about props and he really knows his stuff, so if you can get hold of him, might be worth a go.

There are also some commercial issues you need to be aware of that could be flavouring the advice you are getting. I will send you a PM shortly.

Mark

[Robroy]

Mark

I already called Haines Suzuki today and the guy I spoke to knew his stuff, he agrees that a 14" dia prop would be right way to fix the problem and not to recup the blades as was suggested by the guy I am dealing with.

Also rang a prop specialist/manufacturer in Sydney and he agreed that a bigger dia is better. I have since called and also emailed a DF115 spec brochure to the prop dealer which show all prop sizes to be 14" (3 blade only shown).

Strange though when the first prop arrived I was on the phone straight away asking why a 13" dia and not a 14" just shows size sometimes does matter, I feel if we had started at a 4 x 14 x 17p s/s prop we have reproped my boat ny now instead we are going back and forward an getting nowhere.

Surely all the tests and info I have provided the prop dealer should help them in the long run I get the feeling they wish I had never called (they dont call me Robroy for nothing).


Thanks again for your input Mark much appreciated.


Robrpy

[SeaSaw]

Robroy, Check your PM's ..... it might explain some of the funny stories you have been getting ;)

[Robroy]

Cheers Mark


Robroy