Prop Questions for my Bass Boat

[MD Coleman]

I have a 1997 Sprint Bass Boat that I inherited when my dad passed away a few years ago. I just got the boat out this summer and here are my issues. The boat title says length 16.2, Fiberglass. It is max hp rated for a 120hp and weighs in a little less than 1000lbs dry without engine. The engine is a Force 90hp that weighs approx 286lbs. The boat gets up quick and runs great but the top speed is about 40 max. That is trimmed and half loaded down. Problem is that I can turn about 6k rpm's plus wide open. I have been told that I should not be turning that high of rpms, so I am wondering should I run a bigger prop and if so will it increase my speed as well as control the rpms from going that high? What do folks recommend that I do. I cannot find the pitch of the prop that is on it and I have looked everywhere on the prop. I would like to get as much speed as possible and have considered re powering the boat with a 115hp or 120 if I could find one but the weight difference in a bigger engine may not do much for the speed of the boat. Any suggestions appreciated.. Thanks

 

[QBhoy]

Hi
to make an accurate judgement on a prop for your boat, the usual details are required.
Max rpm at wot, max speed gps, gear ratio, current prop etc as you know, but....
i could hazard a guess at it...I shouldn’t, but I will.
Only because I had a similar boat and the mercury 90. It had a 19” standard mercury ally prop. Trimmed up it would see early to mid 40’s and perhaps a little high on the revs. You might get away with the same or similar
word of warning though. I would push that force too hard. They aren’t known for their reliability, despite being loosely based on the mercury engine.

 

[jimmbo]

There has to be some info on the prop as to what pitch it is, and often the dia is also on there. Places to look... On older style chrysler lower units(non thru hub exhaust props) the info should be on the hub under the prop nut(cone), on the newer engine that use Mercury props, the number is on the outside of the hub, lengthwise on a merc or quicksilver prop. After market 3rd party props, it can be anywhere, you just have to keep looking, with the prop off of the engine.
Before you go adding pitch to lower top rpm, you need to know the WOT range for your engine, and you need to know if the tach is accurate. The general rule of thumb, is an inch change in pitch on a particular prop line/model will result in an approximate 200 rpm change. If you change brands or line/model of prop, all bets are off as to what the difference will be. Different brands/models may include blade design, blade area, cupping, rake, which will affect performance, example, a 20 inch Mercury AL prop, will perform different than a 20 inch Raker, a 20 inch Laser, a 20 inch Turning Point, or a 20 inch Enertia. Not just in top speed, but how the boat rides and handles.
Going from a 90 to a 115 or 120 should result in a a 5 or 6 mph increase, if the rigging and prop lineage is similar. With some tweeking of the rigging a few more mph can be attained.

 

[WesNewell]

http://boatspecs.iboats.com/Force_by_Mercury_Marine__90ELPT__90hp__1997/bpe/57e15567

According to this rpm range is 4750-5250. If that;s even close to right and you know your rpm's is correct, you are way under pitched. A 90hp on a 950# boat should get at least 50 mph and 55-60 is not unreasonable setup for speed. I'm going to guess you have about a 17p prop on it now. I would try a 20-22p prop and go from there.

 

[jimmbo]

I would really like to see a 950lb hull go 50+mph with a 90hp. Most 900lb hulls with a prop rated 90 will do about 40 mph.

 

[WesNewell]

well, if you can't get 50 mph out of a 90 hp engine on s 950# hull then you don't know how to setup an emgine I can 40 mph out of my 995# hull with my 60 hp merc 4s.

• Boat type: Bass
• Usage: Top Speed/0 ft
• Engine Manufacturer:
  Mercury
• Engine Type: Outboard
• Engine Year: 2017
• Number of Engines: One
• Engine Family: FourStroke
• Engine: 90 FourStroke 4cyl (2061cc)
• Drive / Gear Case: Mid Outboard 2.07 Gear Ratio

[h=3]Details[/h]

• Calculated Pitch: 22.53
• Calculated Weight with engine(s): 1,677.40 lbs
• Calculated Engine RPM: 5,750
• Calculated Boat Speed: 53.51 mph

 

[WesNewell]

For the record, my 1983 Kingfisher 15V with a 1983 115hp rude would do 60+mph with a 23p prop. I was a cadet about 60 years ago.