Prop recommendations/suggestions please.

[Barry Friesen]

Hi everyone,
I want to purchase a spare aluminum prop that best suits my boat and setup and need your expertise. We don?t pull skiers or tubes, mostly fish and cruise the lake. Often the boat is nearing the max loading capacity. A friend has suggested that the prop may be slipping excessively.
Here are the facts;
2005 17? Aluminum Crestliner, Fish and Ski.
2004 Johnson 140 hp four stroke.
13.25 x 21 aluminum three blade prop.
GPS indicated speed is 40-41 mph.
WOT is 58-5900 rpm.
Manufacturers suggested WOT range is 56-6200 rpm.
Gear ratio is 2.38:1 (I think, found it on the ?net)
Prop tip to cavitation plate measures ??

I am open to changing the diameter, pitch, going to four blades, or all three! I look forward to your feedback. Thank you.

Barry

 

[steelespike]

Re: Prop recommendations/suggestions please.

You might consider a 19" pitch same brand and style Probably would put you at that 6,200 and improve hole shot and load response.Just watch the rpm with a light load.

 

[Barry Friesen]

Re: Prop recommendations/suggestions please.

I am aware that my rpms would increase with a decrease in pitch. I was told that it is best to stay at the mid point of the suggested rpm range, which is pretty close to where I am now. Is this correct? Also as mentioned, I'm wondering about excessive prop slippage. How is that calculated? How much slippage is acceptable?

 

[steelespike]

Re: Prop recommendations/suggestions please.

In my expierence Aluminum boat performance can vary quite a bit between various brand/styles Yours does seem a little slow though within the the range I have seen.
If you feel your specs are accurate you can check slip with the various prop calculatrors.Bottom design especially in aluminum can exacerbate slip.
You might check your setup. The antivent plate (cavitation plate)should be about
1" above the bottom of the boat.I usually suggest raising the motor to a point just before venting becomes a problem.In extreme cases this could result in 3-4mph.
and improved handling.

 

[Barry Friesen]

Re: Prop recommendations/suggestions please.

OK, so I have raised the motor about 2" so the cavitation plate is now level with the bottom of the boat (I can go up one more hole). This is what I have found.

My RPM's have increased by about 150-200 to 6000-6100 (max is 6200). My speed stayed about the same, maybe a one MPH increase (about 40-41ish). According to the online slip calculators, I have about 20% slip!

I'm thinking of trying a 4 blade 19 pitch. I don't want the WOT RPM's to increase.

Your opinions/suggestions please.

Thanks.

Barry

 

[trendsetter240]

Re: Prop recommendations/suggestions please.

Is your current prop the stock BRP prop that came on the motor when new?

I'd be a little worried that a 19p 4 blade solas amita would put you up to 6200 RPM or higher... In my experience the stock BRP 3 blades turn about 200RPM slower pitch for pitch when compared with the 4 blade amita.

Now that you've got the motor at a better height, I think you could try a 21"p pitch 4 blade on that setup. With 2.38:1 ratio that 140hp motor should be turning a 21p.

It's hard to be exact so it would be best if you could try one out and exchange if it doesn't meet your needs. On the bright side, the solas amita is on 99 bucks here on iboats.com. I've got one and it's a very good inexpensive prop.

 

[1kruzer1]

Re: Prop recommendations/suggestions please.

I agree your top speed seems a little low for that boat with 140HP. Once underway, are you trimming the motor up properly? Trimming up will increase boat speed by several MPH over the full down position. I also second the recommendation on the inexpensive Solas prop. I run a Rubex 3 blade aluminum now as my primary prop, and notice an improvement over the original prop.

 

[Barry Friesen]

Re: Prop recommendations/suggestions please.

When cruising, I trim up the motor until just before the prop vents. I do find at certain speeds, when trimmed up high, the boat seems to porpose easily and will trim down a bit to lessen it.

I could lift the motor one more hole (about an inch) should I try it? I have been thinking about the 21" 4 blade as well.

Trendsetter, I don't believe it is the original prop.

Thanks for your input everyone. Any further suggestions would be appreciated.

Barry

 

[steelespike]

Re: Prop recommendations/suggestions please.

I think you should raise the motor again.The Solas prop will tollerate maximum height.
I think Trendsetter has a better grip on the Solas 4 blade results.
I find slip calculation tricky easily warped by faulty info.The 4 blade will help control slip.

 

[Barry Friesen]

Re: Prop recommendations/suggestions please.

Boat performance report...
First time out with the new prop, I had raised the motor one more hole as well for a total of almost three inches since I started! Now the boat comes out of the hole better, 43 mph (increase of about 3 mph) and WOT of 5750RPM. My prop slip has gone from 19% to 11%. I have gained about 3-4 mph at my cruise RPM of 4000.

I'm happy with the performance increase since I was just out to buy a backup prop! Needless to say but, the new one is now my primary prop. Thanks to everyone for your advice!

Barry

 

[trendsetter240]

Re: Prop recommendations/suggestions please.

Boat performance report...
First time out with the new prop, I had raised the motor one more hole as well for a total of almost three inches since I started! Now the boat comes out of the hole better, 43 mph (increase of about 3 mph) and WOT of 5750RPM. My prop slip has gone from 19% to 11%. I have gained about 3-4 mph at my cruise RPM of 4000.

I'm happy with the performance increase since I was just out to buy a backup prop! Needless to say but, the new one is now my primary prop. Thanks to everyone for your advice!

Barry

So was it the 4 blade, 21p solas amita that you bought?

Cheers