Prop & RPM Question

[playyas4u]

I have a pontoon type houseboat approximately 5500lbs and 8.5' x 30' long with a 1989 85HP Force. I installed a tach last week and took it out on a test run. At WOT it only went to 2900 RPM and idles around 1000 RPM in the water and runs about 7.3 mph WOT. I was thinking about changing the prop out to get the RPMs higher however I can't find a model number anywhere on the prop. So where do I start with picking a prop for this boat. Also if I go down on pitch and increase my RPMs will I pick up speed also? Thanks in advace. Maybe someone can identify the type of prop I have from the picture. Thanks.

 

[Lyle29464]

Re: Prop & RPM Question

I think you have some bad numbers. Did you set the tach for the correct # of cylinders.
are you using a GPS?

If your numbers are correct you must go to a "much" smaller prop. google Prop Calculator and stick in your numbers and the Manufactures recomended RPM for you engine. you will need to re-prop for that RPM. If your engine is not running at full power you will need to fix it first thing. You should not operate the boat till you get it in the correct RPM range

 

[fucawi]

Re: Prop & RPM Question

if your 2900 is really 5800 then the prop is fine.......when you are sure remember that changing the prop by 2 in of pitch changes the rpm by 400 you may do better with a 4 blade prop BUT you must knock 2 in off the 3 blade pitch to get the right one.

All your figures sound like you have something seriously wrong with either the figures or the motor ...7.5 mph and 85 hp ...????? are you trailing chain !!!

 

[SweeperForce]

Re: Prop & RPM Question

I'm pretty sure that motor uses a 20 pole tach. Most newer Forces use a 12 pole. The 20 pole tachs are getting harder and harder to find. I think e-bay may have a few.

Tom

 

[jerryjerry05]

Re: Prop & RPM Question

From all the corrosion/gunk on the lower unit are the pontoons that bad?
That will slow you down a bunch.
The prop should have #s somewhere.Maybe between the blades or under the nut/washer.
Clean the prop and I'll bet their there somewhere.It looks like a pretty steep pitch at least a 19 maybe a 21p.The factory prop has the #s between the blades.Michigan Props make a replacement. Pontoon boats need a much lower maybe a 13 or even lower.
The tack has a dial on the back make sure it's set right.Or better yet get a Tiny Tach part
#79-859000a 1 From Mercury.It will tell you what you need to know as far as RPM's and if your tach is working right.

 

[playyas4u]

Re: Prop & RPM Question

I am using GPS for the speed. The tach is set on 20 pole and I have no reason to believe that it's not accurate. This is a big heavy houseboat so I don't expect to get much over 10mph with this engine. The average I have found with most boats like mines is between 8 - 15mph. The motor is mechanically in good shape. I had a tune up done last year that included a compression test, carbs rebuilt, fuel pump replaced, plugs, etc. The compression was good on the motor. I am thinking that the prop is way over pitched but haven't been able to find a number to verify it. I will re-look for a number on the prop but have looked everywhere already. I am thinking I should be running probably a 10 or 13 pitch. If that prop is a 17 pitch then I am way over pitched for this boat. I cleaned the toons after that picture was taken so they look a lot better now than in the picture.

 

[fucawi]

Re: Prop & RPM Question

30 ft big square house boats on the noosa river had two 9.9hp outboards ...even if it has a 17p it would not hold down to only 2900 .my 90 hp ran a 19 pitch with a good hole shot ...I think you want a hand tacho or a tiny tach to be sure or you cannot calculate the new prop you need....

 

[Frank Acampora]

Re: Prop & RPM Question

If the tach is reading 1000 RPM at idle, then it probably is reading close to correctly at full throttle.

You are pushing a large, heavy, displacement hull and the prop you currently have installed is a relatively small diameter (13 inch) high pitch (probably 17) designed for planing hulls.

You can fit up to a 14 inch diameter prop on your lower unit. Larger diameter equals greater blade area which is important in moving displacement hulls. You need to look for the largest diameter up to 14 inches with the small 4 inch hub offered by manufacturers and low pitch like 13 or 15 pitch to bring up RPM and move more water, thus increasing efficiency of your engine and also probably increase speed some. You would also want a 4 blade prop if offered in large diameter low pitch but may need to settle for the three blade.

Looking at your current prop, there should be numbers either under the prop nut washer or on the flange (if there is one) at the front of the prop (facing bow).

Your engine is a single exhaust model so you can use either an open hub prop (like you have now) or a closed hub model. Small gearcase 15 spline Mercury props will fit.

Until you do get a correct or more correct prop, please do not operate at full throttle. You are severely "lugging" the engine and this can have detrimental effects on the engine such as excessive carbon deposits and loss of compression along with reduced engine service life.

 

[pnwboat]

Re: Prop & RPM Question

If you need a prop, try these folks. Looks like they have a 13 pitch prop that fits your motor. I've purchased several props from them and have been very satisfied with their customer service. Bob is the owner and is very knowledgeable about the props for Force motors. Good guy to talk to.

http://www.midwestpropeller.com/


Here's another site. I personally have not purchased anything from them but they are one of the few places that lists a variety of props for Force motors. I would call them to check availability.

http://www.propcopropellers.com/index.html

Before you make a choice, like the other members have mentioned, you really should find out what pitch you currently have and make absolutely sure the motor is firing on all 3 cylinders. 2900 RPM's at WOT is pretty low.

 

[playyas4u]

Re: Prop & RPM Question

The last 2 posts were really helpful. I spoke with the guy from 'propco' and he recommended the reverse cup 14x10 prop for my application. The reverse cup will help with backing off islands, which is always a difficult task. He also agreed that I need to go with biggest diameter possible which is 14. Thanks for everyone help. I will let you know how it goes after I get the prop.

 

[playyas4u]

Re: Prop & RPM Question

Will a prop from a Mariner 60 Big Foot engine work with my engine? The part number for the prop is 77338 and it's a 14x11. I can get a great deal on it, so if it will work I may just try it out until I can get the one from Propco.

 

[Frank Acampora]

Re: Prop & RPM Question

mariner is a Mercury brand. If the prop for the bigfoot 60 has a 4 inch diameter hub, the necked down portion is about 3 3/4inch, and is 15 spline, it will fit with perhaps a small amount of modification.

 

[playyas4u]

Re: Prop & RPM Question

I will have to get the guy to do some measurments and count the splines but I am pretty sure it's 15 splines. Here's a picture of it if that helps to tell weather or not it will work.