Assistance with Prop?

[rdrainvi]

I'm running a '96 115 HP Merc 2 stroke with a stock 21P AL prop on a 17' BW Montauk. With 4 adults aboard, at 5,000 RPM, my speed on GPS is 30 mph. Mileage is 5 mpg.The drive is trimmed down to maintain even plane. I have no trim tabs or hydrofoil. I expect a prop change would improve this poor performance, but I'm not sure which way to go. Suggestions would be greatly appreciated. Thanks, Jim

 

[RicMic]

Re: Assistance with Prop?

I had the same boat with a 115 Johnson and it would do 50+ with just me onboard and a 19" SST prop. I don't know how much weight you have in there, but it still should do far better than 30 with 4 adults, unless they are really big adults. Tabs or hydro are not going to help speed or mileage much. If that is your normal load, you do need to drop in pitch and I don't think it would hurt anyway to go to 19"SS. Is the bottom clean?

 

[steelespike]

Re: Assistance with Prop?

To get an indication of your real max speed you need to trim out to the fastest rpm and speed combination.If your trimmed in for level smooth ride you likely have too much boat in the water. Also your antivent plate(just above the prop) should be visable on plane.If not then you need to raise the motor.In the extreme this alone could result in 2 to 3 mph and more rpm.
Your max rated rpm is 5250. I don't know if they recommend higher due to todays doctored fuels. A 19" prop,same brand,same model would increase rpm
"about" 300 to 400 rpm.

 

[rdrainvi]

Re: Assistance with Prop?

Thanks for your comments. The 4 adults referred to are maybe 600# total, with weight evenly distributed. The bottom of the boat is clean. My trim is max as the stern sits low with that Merc so I realize I'm sacrificing some speed but 30 mph is far less than I expected. From all accounts in the forums I ought to be at 40 mph minimum. My antivent plate is just visible on plane. I have some throttle left, but it's not going to improve this situation much and I don't want to run much over 5,000. I'm prepared to invest in another prop as it seems to be the problem here - if there is a particular prop that would deliver the expected performance under these conditions, I'm open to suggestions. Thanks again for the help, guys. Jim

 

[trendsetter240]

Re: Assistance with Prop?

I'd try a Solas HR Titan 4 blade stainless prop for that setup. The HR Titan has very smooth power and excellent stern lift. The stern lifting properties of this prop will help to get more of the boat out of the water and reduce drag.

I believe a 19p in this prop will provide the correct RPM and best cruising economy.

I use a 15p of that exact prop and I absolutely love it. A bit pricey compared to aluminium props but well worth it in my oppinion.

They sell them here : http://boatpropellers.iboats.com/Me.../7306/?**********=004026292&*******=358654179

 

[RicMic]

Re: Assistance with Prop?

My trim is max as the stern sits low with that Merc so I realize I'm sacrificing some speed but 30 mph is far less than I expected. From all accounts in the forums I ought to be at 40 mph minimum. My antivent plate is just visible on plane. I have some throttle left, but it's not going to improve this situation much and I don't want to run much over 5,000. I'm prepared to invest in another prop as it seems to be the problem here - if there is a particular prop that would deliver the expected performance under these conditions, I'm open to suggestions. Thanks again for the help, guys. Jim

You really need to have the RPMS at full throttle and a normal to light load, even if you don't ever intend to run at full or WOT, to pick an ideal prop, you need to have those numbers. I'm not sure what you are saying with your trim at "max", max what? Whalers have a huge amount of lift, so are very trim tolerant.

 

[rdrainvi]

Re: Assistance with Prop?

At WOT I'm about 5,350 RPM. The speed gain beyond 5.000 RPM is negligible - about 1.5 mph. By "max" trim, I mean the drive cannot be lowered any further. 3-4 adults would be my "normal" weight. I'm not carrying any other weight, beyond 10 gallons of fuel and required safety equipment. Hope this clarifies my description. Considering a 4 blade prop?Thanks.

 

[RicMic]

Re: Assistance with Prop?

You really shouldn't ever have to have that motor trimmed all the way in on that boat, unless all your passengers are in the back. I had as many as 7 or 8 people in mine, some in the very bow and it still planed out and ran well. Something seems off, turning that many rpms with that prop, you should be going faster than that, changing props isn't going to give you another 10 mph. A friend of mine had that exact same motor and it would do around 50 mph on his lightly loaded Montauk with a three blade al Merc. prop, although I don't know what pitch. Did you buy that motor new?

 

[steelespike]

Re: Assistance with Prop?

At WOT I'm about 5,350 RPM. The speed gain beyond 5.000 RPM is negligible - about 1.5 mph. By "max" trim, I mean the drive cannot be lowered any further. 3-4 adults would be my "normal" weight. I'm not carrying any other weight, beyond 10 gallons of fuel and required safety equipment. Hope this clarifies my description. Considering a 4 blade prop?Thanks.

If your trimming in all the way there is no telling where your speed will be.
Certainly not very good.In the early 1900s they thought the way to design boats was long and narrow and push them "through" the water. They soon found out that to gain
5 mph they had to double and triple the power and the boats were impossible to handle.
Then some genius said lets put the boat on top of the water and suddenly they had
25 hp boat that was as fast as a 100 hp boat.In those days a 100 hp probably had 700 cubes and weighed probably a 1500 lbs or more..Needless to say you needed a big boat.
Your 115 is capable of 40 or more but not with the whole bottom in the water.

 

[rdrainvi]

Re: Assistance with Prop?

Exactly. Something is quite off. Like adding 2+2 and getting 3. The OB is a 96was used very little and runs well. I just bought the package from the original owner. The prop is OEM as far as I know - PN # 48-32746A3 21P. If the prop is slipping significantly would a more aggressive prop help? Thanks.