Need help - Prop sizing and current low rpm's

[anthony1]

Hi Guys

Heres the skinny - 28' catamaran. Twin 60hp merc's 4 stroke 2006 with the bigfoot.

Problem - only 4100 on the wot for both motors, motors making a little oil, might be low rpm related or not I will determine that this week I hope.

Current prop numbers on the prop( I have not figured out pitch and Diam yet)
#4877342a33 15pl

Boat is casual use twice a week about an hour.

I am interested in replacing both these guys as the numbers are not adding up. I cannot find the weight of the boat as they are out of manufacture and I have yet to find the name plate.

I was thinking of going with the piranah type of prop. But really I am looking for some experteze suggestions here?

Please Advise

 

[Silvertip]

Re: Need help - Prop sizing and current low rpm's

You have 15 pitch props. You more than likely are killing your engines because they are over propped by a bunch for the length of the boat (cat or not). You should be in the 13P range and possibly even less. You might consider a pontoon-specific prop for this application. Pontoon props have very large rounded blades that provides lots of push and they also work much better in reverse than a standard prop.

 

[anthony1]

Re: Need help - Prop sizing and current low rpm's

Thanks Silver tip --- Yes I figured they are killing the motor slowly....

The boat is not a pontoon its an Hawiian seacat built for the pacific ocean. Do you think the pontoon props would still apply. I have attached a photo

I have also been thinking about piranahs because I can adjust the pitch and of course the cheap blade fix if you knacker one...

 

[steelespike]

Re: Need help - Prop sizing and current low rpm's

First thing I notice is assuming the tachs are accurate you have to make up 1,900 rpm.
thats over 9" of pitch,not possible.Something is up.If 2 tachs both at 4100 I guess we can assume they are set the same. are they set right?Sail boat looks conventional
I'll assume the closer boat is yours.
Is the boat intended to plane?Does it plane? If intended to plane and it doesn't make it then a slight prop change might get it there?What is rpm and speed.

 

[Silvertip]

Re: Need help - Prop sizing and current low rpm's

You have two motors on a 28 foot boat. Thats a lot of work for two 60's regardless of the hull type. The very fact you have Big Foot motors says the selling dealer knew what he was doing. It would have been a greate disservice to sell standard lower units for this application. Yes - it is push you are after so a pontoon prop still applies. If you don't understand the difference between the two lower units, the Big Foot is geared differently and takes a larger diameter prop than the smaller standard gearcase. Big Foot motors were designed for pontoons, work boats, and any application requiring lots of push where speed is the lesser consideration. And no -- you can't have it both ways. And yes -- prop selection is a crap shoot but you definitely have the wrong ones. You need to look at the label on the engine block to see what the WOT rpm range is for these engines. I think it's in the vicinity of 6000 RPM so you can imagine how wrong the props are. To pick up 1900 you would need to be down in rediculously low pitch numbers which raises another question. Have you verified that the tachs are correct. If they are, then you are severely under powered for this hull. To gain 1900 RPM you would need to drop pitch (theoretically) by 9 inches leaving you with about a 6P prop. That is not practical and there probably isn't a prop available for your engines with that pitch. A dirty hull may also be causing extreme drag so before you spend money on props, you best dig into the tach and other performance robbing characteristics first. Lastly, because the engines are struggling, a 10 or 11P prop may get the engine up on the torque curve so it can actually accelerate and reach the upper end of the rpm band. Getting "over the hump" is a comparison that comes to mind. I believe you need have 10P or 11P props on this rig. Since you are buying two, I would try to find a shop that will work with you on a try it and buy it process. But be a sport and buy from the dude if you find something that works.