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Discussion Starter · #1 ·
I’m new to boats;

I have a 17’ Lund with a 90HP Honda 4 stroke and have a hard time getting the boat up on plane and keeping it there.

I have to trim the motor as far under the boat as it will go and even then if I slow down the bow comes up.

The folks at the local boat supplies and repair shop said that a different prop with a faster pitch will solve the problem, other people suggested trim-tabs.

Any advise would be welcome.
 

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Are you hitting your WOT rpms?
 

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You don't want to start changing props without knowing the impact on several factors including the maximum RPM of the motor. Before anyone can help you, first we need to know what RPM you are running when wide open with just you in the boat, what the pitch is on your current prop, and what the maximum RPM range is for the motor at wide open throttle. When you know that, come back here and maybe someone can help. Other wise you will get a bunch of responses about moving weight around or other guesses that won't really help.
 

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agree with flinch

mtor rpm's

range at wot(specs)

prop? blades/pitch/alum/kind

what hole motor is mounted in?

get this info and myslef or soemone may be able to point into right direction
 

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1fife said:
agree with flinch

mtor rpm's

range at wot(specs)

prop? blades/pitch/alum/kind

what hole motor is mounted in?

get this info and myslef or soemone may be able to point into right direction
Some good points. And while we are on the subject, let us know the condition of the current prop, are there dents, is it like new, etc. I am also curious to know what the maximum horsepower is for the hull although it won't help with prop selection. That will be on the same plaque with the maximum weight and passengers.
 

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Discussion Starter · #7 ·
The prop is a 25X17 and in decent condition. Just unwrapped it tonight I'll take her for a spin Saturday and get back with the RPMs.

Not sure what WOT means? Does that refer to the max wieght of the motor I can put on the boat?
 

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WOT is Wide Open Throttle. It refers to the manufacturers recommended RPM rating with the throttle open all the way. I could be wrong but 25 X 17 sounds like a LOT of prop for that setup. I'm not too familiar with four strokes but would assume that they run at a lower RPM than two strokes. But 25 X 17 is a lot of prop!

John
 

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AGreed.... 25 x 17 is too much prop for 90 hp and a built like a tank Lund 17 footer. This would go a long ways towards explaining why you can't keep the bow down at lower speeds.

My guess is your rpm's are low at WOT. The RPM range for your motor is 5000-6000 rpm. Max HP is acheived at 5500 rpm. ( http://www.honda-marine.com/modelDetail.aspx?modelGroup=BF90 ).

Here are some prop tests done by the manufacturer using a couple different boats. You have WAY (did I mention WAY!!) too much prop. http://www.honda-marine.com/performancetest.aspx
 

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Ok that's kind of what I thought but wasn't sure. I've never looked at the specs too closely on four stroke motors because I've only owned two-strokes. It looks like the RPM specs are pretty much the same. If that's the case then I'm sure that he's just got far too much prop there. I would guess that a 17 or 19 max should be a about right.

John
 

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25 X 17???
Usually props are listed with the diameter first and the pitch second. I don't think a Hondo 90 can swing a larger diameter than 14".
I would think a 13 X 17 would be just about right, maybe a 19 if you don't run around with a huge load.
 

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I have a 17' lund with a 115 4 stroke merc. Spinning a 13x17 prop with 17 being pitch. Good hole shot and top speed of 40 mph at 5400 rpms with 2 guys and all our stuff.

Greg
 

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a hint do not trim motor all the way down, this will force the boat to dig and take you off plane. sounds like you might have a bigfoot lower case wich is made for flatbottoms and pontoons. not much of a problem can still get the boat to run. keep motor trimed in as you floor the gas than slowly start trimming out tell you see water pressure fall off than slightly trim in this is were we need a reading of wide open rpms
 

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Northbound said:
The prop is a 25X17 and in decent condition. Just unwrapped it tonight I'll take her for a spin Saturday and get back with the RPMs.

Not sure what WOT means? Does that refer to the max wieght of the motor I can put on the boat?
That is not the correct prop size. You are saying that either the prop is 25 inches across with a pitch of 17 or is 17 inches across with a pitch size of 25, neither of which is correct. Just write down all the information on the prop and we will look it up. Usually the full model number information is stamped on the prop somewhere. Then get out and run the boat and tell us the rpms you are running at wide open throttle. I would still like to know the maximum horsepower for the hull. Also tell us the speed you are running at wide open throttle, either from the speedometer or preferably from a GPS unit.
 

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I don't think that a 25" DIA would fit on that motor. Heck, I don't think a 17" DIA would either.

13x 17 sounds like the one to have on there, if I had to pick one off the top of my head.
 

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When checking RPM it's best to use an accurate Engine Analyzer (Dwell Tach with a large dial). Dash tachs can be off as much as 10%. It's difficult to determine the exact RPM, and stay with in a 500 RPM range, with a dash tach that has scale marks every 200 RPM.
 

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Discussion Starter · #17 ·
Ok, I believe I have all the info

Prop; Vortex MI Wheel 992104 13.25X17

Max RPMs with just me in the boat is 2900

Speed using a GPS was 34.6 MPH that is with adjusting the trim to maximize the speed and an 8 MPH wind at my back.

I believe the hull is rated for better than 125 HP, I originally wanted 115 HP and the dealer talked me out of it.

The max weight of an outboard for the stern is 570#

The out board I have (90 Honda 4-stroke) weights 394#

Any suggestions?
 
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That prop does seem a little large for a 90. The RPMs are low at full throttle. A small prop would get you out of the hole quicker and make it much more responsive. Still, I would guess that even if you change your prop that you will still have nose up issues. Many boats are just ass heavy as they have the motors, batteries, gas tank and people twards the back. Many people have to add weight to the front of the boat. I know of one I have a couple sand back in the front of my Lund and I know others that have done the same thing. If your boat as forward livewells, try filling them and seeing how it rides. You also might want to add trim tabs. I added them to my Lund an they really help to keep the bow down and level the ride if you have a canting problem. Tabs allows you to tilt the motor to a more effecient angle and still keep the ride good. They save fuel that way. If you don't want to add tabs, then one of those motor attached foils like the Stingray might help. I had one on my last rig and it worked great, but I did hear that Mercury will void the warantee if you add one to their motor.
 

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13.25x17 sounds about right to me. Orignally when you said 25x17 I figured that you had just reveresed the diameter and pitch. It didn't even dawn on me that 17 was way to large of a diameter for that motor either.

I wonder if it isn't more of a tuning, carburetor, or throttle problem as opposed to a prop issue. 2900 is way too low of an RPM to be running at WOT. I'd check into those other areas too. I would think that the prop on there sounds like it's pretty close to the correct one.

John
 

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Some thing still isn't right with your numbers. That motor on your boat turning 2900 R's would go about 18 MPH. Are you sure about your RPM's
If you are getting 35 mph you should not be having a planing problem.

Review your numbers again and re post them, som thing just isn't right.


Northbound said:
Ok, I believe I have all the info

Prop; Vortex MI Wheel 992104 13.25X17

Max RPMs with just me in the boat is 2900

Speed using a GPS was 34.6 MPH that is with adjusting the trim to maximize the speed and an 8 MPH wind at my back.

I believe the hull is rated for better than 125 HP, I originally wanted 115 HP and the dealer talked me out of it.

The max weight of an outboard for the stern is 570#

The out board I have (90 Honda 4-stroke) weights 394#

Any suggestions?
 
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