What is the theoretical speed of my boat?

[mbhoag]

OK, I have checked out several speed calculators and propeller slip calculators... but none of it makes sense to me... I understand the most basic of theories... but when I put in my numbers, I somehow come out that my boat should do 72 MPH....

That only happens on the freeway when I am towing it....:facepalm:

So I know if my boat is even close to running the way it should... what SHOULD my theoretical speed be?

Here are my boat specs...
1986 Bayliner Fish & Ski Trophy Bass Boat (17' 10", 6' wide) (Fiberglass)
Max load with 4 people, motor and gear - 1200 lbs (according to dataplate)
I have no clue what the actual boat weighs...
1986 Force 125 motor - 125hp @ 5000 rpm
Operating range 4500-5500 rpm
fore & stroke - 3 5/16 x 2.87"
Gear Ratio 15.26
Engine Weight 300
Propeller - Stainless Steel, Michigan Prop, 3 blade, 15P

So with all this info, what should the boat be able to do?

Thanks!

 

[Frank Acampora]

Re: What is the theoretical speed of my boat?



At an engine speed of 5000RPM with a 15 inch pitch prop, your 1.73 ratio lower unit, and a theoretical 10% prop slip, your theoretical top speed would be about 40 MPH. At 5500 RPM it should be about 44-45 MPH. To figure it yourself, take engine RPM and divide it by lower unit gear ratio (1.73). Multiply by propeller pitch and divide by 12 to get feet per minute. divide by 5280 (1 mile) and multiply by 60 to get miles per hour

Now, If your engine can swing a 17 pitch prop then: At 5000--46-47 MPH and at 5500 -- 52MPH---never gunna happen! If you are getting up to around 47 MPH with that engine, you are doing well.

This 14 footer with a 125 like yours and a 2 to 1 ratio lower unit, swinging a 21 pitch prop at 5900 RPM topped out at 60 MPH. Obviously, there is a lot more than 10% prop slip.

 

[saumon]

Re: What is the theoretical speed of my boat?

You need to recalculate it with a gear ratio of 1.73 instead of 15.26!

According to the manual for your particular engine (http://reinsmith.net/gallery/boating/documents/Force_85-125_HP_Outboard_Owners_Manual.pdf), the gear ratio is 15:26 (it's written backward, so you need to divide 26 by 15, which give 1.73:1, or simply 1.73).

Other than theorical speed, what are your actual top speed and RPM at WOT with your 15p prop and a normal load?

 

[Frank Acampora]

Re: What is the theoretical speed of my boat?

The 1.73 ratio on that engine is regressive. The engine performs MUCH better with a 2 to 1 lower unit and a prop pitch 2 inches higher. Spinning the higher pitched prop slower through the water produces less parasitic drag and yields more top speed.

 

[dan02gt]

Re: What is the theoretical speed of my boat?

Most 17' Fish and Skis with that kind of power will get you in the mid to upper 40's. But the motor, prop, and hull must be in good shape for you to get there.

 

[mbhoag]

Re: What is the theoretical speed of my boat?

Wow... my head is spinning trying to follow and make sense of pitch, slip, prop, ratios, drag... arrggghhh!

Last year (with carbs sluggish and not adjusted correctly, I have since rebuilt them, timed and synced) I was running the boat at 43 mph (according to GPS on my phone). It was a little choppy that day, which probably slowed me down. Have not clocked it since then. Most of the lakes I fish here are 'no-wake' lakes, so I am wondering what improvement I might see now that all is running at 100%...

Sounds like unless I change to a 17 pitch, I may already be at my top end.

OK, let me ask a reverse question... since I fish a lot of 'no-wake' lakes, and I am starting to do more trolling, which way would I go with prop pitch that would help me at slow speeds? I really should have 2 props anyway... might as well make them different!

Currently at about 800rpm idle speed, she moves at arould 3.5 mph (according to my new lowrance unit) and I would like to slow it down but keep rpms where engine does not want to foul out and stall occassionaly.

 

[saumon]

Re: What is the theoretical speed of my boat?

Changing to a higher pitch to improve top speed or to a lower pitch to reduce trolling speed should NOT be done, EVER, it will hurt your engine, reducing its life and, anyways, won't work (lugging an engine or, on the opposite, over-revving it aren't good at all). Prop pitch must be matched so engine reach its optimum rpm at WOT with a certain load (that's the key words). When you find a prop that allow the engine to reach that peak rpm at WOT, the only circumstance when you'll need to change it was if the load vary, ex: pulling a tube or a skier, so you'll drop in pitch...

 

[mbhoag]

Re: What is the theoretical speed of my boat?

Thanks, I appreciate the info. Guess I should just leave it alone! Guess I should just get an inexpensive prop of the same diameter and pitch for a spare.

Will let you know what I actually get for speed at WOT as soon as I can get it on a fast lake!

Thanks!

 

[saumon]

Re: What is the theoretical speed of my boat?

Before buying another prop as a spare, take time to monitor your actual rpm at WOT with a normal load. If it isn't between 5000 and 5500, you may want something else than a 15p. That way, you would keep your actual 15p as an "emergency spare"...

 

[mbhoag]

Re: What is the theoretical speed of my boat?

Before buying another prop as a spare, take time to monitor your actual rpm at WOT with a normal load. If it isn't between 5000 and 5500, you may want something else than a 15p. That way, you would keep your actual 15p as an "emergency spare"...

Thanks, I will definately do that!