3 blade prop, changing to 4 blade, change pitch?

[roy w. shakun]

Situation: ventilation<br /><br />18 foot cat, yamaha 90, single engine application.<br /><br />current prop, 13 by 17, 3 blade ( solas ), WOT is right at 5500 rpm.<br /><br />New prop, 13 by ???, 4 blade ( solas )<br /><br />anyone that knows about 4 bladed props? ( solas makes a 13 by 13, 15 and 17 in a 4 blade )

 

[Spidybot]

Re: 3 blade prop, changing to 4 blade, change pitch?

Roy,<br /><br />If you feel happy with Solas, going for a 4 blade will improve grip.<br /><br />I had severe problems of the same nature and fixed it all with a 5-blade Quicksilver HighFive.<br /><br />Here's the theory:<br />Regarding pitch you'll lower your rpm by approx. 200 for each inch in higher pitch. As your existing prop vnetilates you have to detect if some of your WOT rpm is a result of that.<br /><br />If not, the same pitch will result in the same rpm and you'll notice the difference only when accellerating + cornering + trim.<br /><br />A testdrive is the only way to tell how it actually behaves with a certain setup.<br /><br />Booklet 'Everything you need to know about propellers' is available for (free) download here http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=17;t=014920 and will tell you al lot about props, hulls, theory, practise...

 

[Dhadley]

Re: 3 blade prop, changing to 4 blade, change pitch?

Since you are sticking with the same brand you should be able to stick with the same pitch IF your at the correct top RPM's now. You may find you can play with the X dimension to get even better performance. Keep in mind that 3 and 4 blade props seldom run best at the same height.<br /><br />Good luck!

 

[roy w. shakun]

Re: 3 blade prop, changing to 4 blade, change pitch?

The ventilation is primarily during accelerration and in a following sea. There is a lot of turbulence between the hulls of a single engine cat,,,,,,,, The 4 blade will have more grip, agreed, thankyou<br /><br />So, if I understand, a 4 bladed prop of the same brand, ( solas ), and same dia/pitch will turn the same revs as a 3 blade?<br /><br />Isn't there more drag with the 4 th blade, decrasing the rpms @ WOT vs the 3 blade?<br /><br />Thanks

 

[Dhadley]

Re: 3 blade prop, changing to 4 blade, change pitch?

Also more blade area therefore the 4 blade can be mounted higher which it sounds as if you may be there already.<br /><br />Let us know how you make out!

 

[Spidybot]

Re: 3 blade prop, changing to 4 blade, change pitch?

All experience from others must be taken with a grain of salt. <br />Your actual setup is special and some experiments nedd be done.<br /><br />If your problem is, that a 3-blade at ant time has too little blade area in the water, a 4 or 5-blade ofcourse will improve.<br /><br />If your pitch is correct (the prop is not slipping heavily at WOT and you're within rated rpm range), best guess is that the same pitch with more blades will end at same rpm.<br /><br />The drag theoretically would change from more blades, but this is a minor issue and is better established by testing as more parametres are involved in your case. If you're buying the prop from a good dealer, he should help with lending a prop for testing.<br /><br />You also have to decide how you expect the prop to perform (traction/hole shot or topend priority).

 

[roy w. shakun]

Re: 3 blade prop, changing to 4 blade, change pitch?

It's all about function<br /><br />Fishing boat<br /><br />Cruise speed<br /><br />Minimal, ventilation in all types of seas<br /><br />Thanks for the reply's<br /><br />Let you know

 

[Spidybot]

Re: 3 blade prop, changing to 4 blade, change pitch?

I never tested Solas (they never delivered a prop I ordered). I got the HighFive and it solved any kind of problem related to grip. Before it would rev to 5000 before the boat moved producing a sea full of foam, loose grip in even the widest turns and allowed no trimming at all.<br />Now it grips perfectly, holds in all manouvers and lets me trim all I need.<br /><br />Based on this experience I'd go for a multiblade prop. I guess the rated rpm at WOT is 45-5500 so if oyu prefer cruising speed to pulling skiers I'd go 2-3 inches up in pitch to land on 5-5100 rpm. Maybe even more if you do not carry a lot of gear / weight.