Trying to figure correct prop

[klauricella]

I have a Wellcraft Coastal 210 with a 1989 Johnson 225, it currently has a 15x17 prop which I think is too big. At 3950 rpm I am getting 30mph.The highest rpm I can get is 5000 at WOT and am going 35mph. I pretty sure that the motor needs to be raised at least one ole maybe two but want to get the correct prop before are start that. Also use it mostly for diving so we are always hauling alot of weight.

Thanks
Kevin

 

[levi_tsk]

Re: Trying to figure correct prop

you actually need to get the motor at the correct height before you try propping it out
you can try the prop wizard here:
but it wont be accuate until you get the height right
http://dansdiscountprops.com/BuyNow/propWizard.cfm
this thing say 15X15 but if you hauling more than a couple sets of gear you need to go smaller you want an unloaded with a full tank of fuel WOT rpm of 5800 that way when you load it down its still turning pretty high

 

[wilde1j]

Re: Trying to figure correct prop

The right height is when the AV plate top just runs dry @ WOT. Once you get that right, try a lower prop pitch. You will probably find that you will gain some speed and RPM as the motor comes up.

This whole deal is an empirical process.

 

[bernd64]

Re: Trying to figure correct prop

14.5X19 3 Blade or 14.5X 18 4 blade props a for this lengt of boat

 

[levi_tsk]

Re: Trying to figure correct prop

^not if hes hauling 4 sets of dive gear 4 adults and 8 full tanks ...... thats almost 1200 lbs....

 

[klauricella]

Re: Trying to figure correct prop

Thanks for the advice, I will raise the motor up first. Right now the cavitation plate is even with the hull two holes would bring it up 1.5". Not too concerned about top speed just want to make sure my mileage is good, seems to be kind of slow out of the hole getting on plane even when empty. Was hoping to have someone with about the same boat and motor let me know what they are using.

Thanks
Kevin

 

[wilde1j]

Re: Trying to figure correct prop

When you motor height right, you will find you will get better fuel economy, top speed also. This is BC you will have less wetted surface in the water (lower drag). Raising the motor first is the only logical way to proceed. You want to shoot for ~ 5800 RPM @ WOT when you're done.

You will eventually end up with a lower prop pitch, which will help the hole shot a bunch, plus your motor will last longer.