new prop or trim tabs ?

[Northbound]

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.

[SalmonBum]

Are you hitting your WOT rpms?

[flinch]

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.

[Northbound]

Will do, I'll post that info in a couple days.
Thanks

[1fife]

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

[flinch]

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.

[Northbound]

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?

[jpollman]

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

[waterfoul]

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

[jpollman]

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

[ESOX]

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.

[greg123]

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

[wildcoy73]

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

[flinch]

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.

[SalmonBum]

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.

[Priority1]

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.

[Northbound]

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?

[Trophy Specialist]

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.

[jpollman]

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

[Lucky Dog]

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.


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?

[wildcoy73]

Need To Have Your Tach Checked Sound Like It Might Be Set Up For The Wrong Engine Most Marine Tach Will Have Sevral Set Ups On The Back One Is For 2 Stroke Outboards And The Others Are For 4 Strokes Need To Get This Checked Out 2900 Should Be Mid Throttle Cruising Not Wot.

[FishTales]

Prop; 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.
The out board I have (90 Honda 4-stroke) weights 394#
Any suggestions?
Just for comparison, I have a 17 ft tracker, merc 75 hp 4 stroke,
Prop 13.75 x 17
Speed approx 32 by speedometer, didn't check with gps.
RPM 4200,
I know rpm doesn't sound right, should get 5,000 to 6,000.
I think I need to go with a 13.75 x 13, that should get my wot up a little.
With the 17 in pitch, boat won'd idle slow enough to troll, I go about 2 mph by gps, trolling speed, also a 13 in pitch would help slow it down for trolling.
Rich

[flinch]

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?

Like lucky dog said, somethng isn't right with your numbers. There is no way you would be doing 34 mph with the motor only turning 2900. That motor has a gear ratio of 2.33 to 1, so with a 17 pitch prop, the maximum you could be doing is 20 with no prop slip. Also, if you were really doing 34 you wouldn't be complaining about the boat barely staying on plane because you would be moving right along.

Here's my educated guess even though more information is really needed. I think you really are turning somewhere around 2900 rpm and the boat is barely up on plane and doing 16 or 17 mph. The reason for this is not the prop. If the boat could just get fully up on plane, the motor would probably push the boat along at somewhere in the low to mid thirties turning at least 5,000 rpm. But it can't quite get the boat up fully on plane because it doesn't quite have the torque for the given hull. You said the hull is rated for at least 125 but you have a 90. You would probably have been ok with a 2-stroke but not a 4-stroke. Oh, and I think your GPS is messed up.

So your options are to try either a lower pitch prop or put trim tabs or something on it. You could do this but the real issue is that you don't have enough motor. Just be careful if you go to a lower pitch prop, 15 pitch for example. It is possible that the motor will be able to get the boat fully up on plane but then it may overrev the motor once the boat gets moving.

It's really a shame that these dealers sell boats this way. When I bought my current 16 foot starcraft the standard motor was a merc 2-stroke 50. I was happy that I could still get a 2-stroke, which is what I wanted, but thought that the boat was a little under horsepowerd since it was rated for 75. I asked the dealer about it and he said it would definitely bog down with a couple people and gear, so I went with a merc 75 2-stroke.

[Northbound]

The tac and speedometer both seem to be off; @ 2900 RPMs the speedometer reads 28MPH, yet, the GPS says 34.6.

So if the prop is the correct size then trim tabs? Or wieghts in the bow?

[jpollman]

I never trust boat speedometers to be even close to accurate. But the tach should be very accurate. Maybe it has an issue as stated earlier.

John

[Trophy Specialist]

The tac and speedometer both seem to be off; @ 2900 RPMs the speedometer reads 28MPH, yet, the GPS says 34.6.

So if the prop is the correct size then trim tabs? Or wieghts in the bow?
Try weight in the bow first and if that doen't do it then get tabs.

[Lucky Dog]

The tac and speedometer both seem to be off; @ 2900 RPMs the speedometer reads 28MPH, yet, the GPS says 34.6.

So if the prop is the correct size then trim tabs? Or wieghts in the bow?

I think before you do anything you need to get your Tac fixed.

17 foot Lunds have been rigged with 90 Honda's for a long time, the Honda's are a bit sluggish, but your motor should perfom pretty good on that boat.

Some Tac's have a switch on the back to set to the number of cylinders your motor has, make sure yours is set to the correct number.

Forget about the speedo, they are seldom right.

I can't understand how your boat can be going 34(gps) and not be on plane? Maybe you could explain more what is going on? Maybe I'm just not reading your post right.

Is this new boat? or a new used boat?

[waterfoul]

Maybe someone needs to go for a ride with him. An experienced boater who has set up boats in the past.

Drag in on over to West Michigan and I'd be happy to give you my help and opinion. :D

And boat speedos are NEVER right unless you pay the big bucks for a Gaffrig or Livorsi. The Gaffrig in my Baja was within 1 mph of my GPS at 60 mph. Pretty dang close if you ask me, and it should have been for nearly $200. BUT, the speedo in my Tracker is about 4-5 mph happy compared to GPS. Boat says 34-45. GPS says 30. And the tac doesn't work. :(