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Propping a Honda BF75 on a Montauk
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| Paul Graham |
Posted on 09/06/09 - 6:30 PM
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The prop I have is an Attwood 335132, 13 3/8 x 19. The spline on Honda's, Yamaha's and Mercury's are the same, therefore you can interchange props if you have access to them. I wanted to try a 17" prop just to see how close it would get me to where I need to be. I used to run 35mph on my 1970 hull with this motor and prop, today with the 17 prop I could tell there was less top end, I can only imagine that I might rum 30 mph with a 15" prop,if that, that's a little slower than I would like. I was pretty happy with 35 mph before. I realize I'm gonna get what I get. It is a little disappointing to see this much of a difference between the two hull's
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| Tom W Clark |
Posted on 09/07/09 - 11:01 AM
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Paul,
OK, I presume the prop you tried is the Painted Stainless Steel propeller from Yamaha. It is a good model of propeller with not a lot of rake and a semi-cleaver blade design.
[ Aside to Joe: The Yamaha 130 and the Honda 75 are both intermediate size gearcase motors and both use the same splines on the prop shaft so you can interchange them. ]
The only time I recommend re-pitching a prop is when you need a one inch change on a prop that is only offered in different pitches in two inch increments. Yes, you can change pitch by two inches, but that does not mean it is a good thing to do. You will not end up with the same prop that the manufacturer makes in the different pitch, you wind up with a compromise that you will not be able to sell later on if you need to.
The guy at the prop shop didn't think it was a big deal because he was the guy who would get the money you would spend to do this.
The Stiletto Advantage model (which I presume is what you started with) is unusual in that if will perform as if it has two more inches of pitch in comparison to most other props.
So it's really like you started with 21" of pitch with the Stiletto. The switch the Yamaha K Series 17" should have resulted in a bigger difference.
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| Joe Kriz |
Posted on 09/07/09 - 4:55 PM
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Paul,
There is another variable here for your engine speed that is not directly related to the difference between the older hull and the newer hull.
This other variable is the "Jack Plate" you just installed on your newer hull.
If you were to mount the engine exactly like it was mounted on the older hull, then we could compare the differences between the 2 hulls.
As it stands now, we have to take into consideration that you also added a Jack Plate to your new hull which changes the calculations and evaluations...
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| Tom W Clark |
Posted on 09/07/09 - 5:16 PM
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I agree with Joe. Yo could still have the motor too low.
At any rate, an Attwood 335132, 13 3/8 x 19 is a Michigan Wheel Ballistic model. Even on the old hull, at 5200 RPM that prop was too much for the motor.
Given the RPM drop sine then, you will really need to drop pitch and even a 15" Stiletto is not going to be enough because it will perform like most other manufacturer's 17".
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| Tom W Clark |
Posted on 09/07/09 - 5:20 PM
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Ad the cause of the difference between the old hill and the new one, you also have to consider the effects of a waterlogged hull. Added weight will kill boat speed...and RPM.
Do you have photos of the new boat floating?
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| Tom W Clark |
Posted on 09/07/09 - 5:21 PM
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And...are we sure there nothing done to the motor, or rigging when the switch over was made? Something that might limit throttle travel, perhaps?
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| Paul Graham |
Posted on 09/07/09 - 5:41 PM
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Tom, you are correct, it was a painted stainless steel Yamaha prop, it seemed to give me about an additional 500 rpm. It is a very plain looking prop, the stilletto seems like the edges have a slight curl, is that cupping. I am convinced, I need a new prop, would you mind telling me what you recommend. I don't see how my motor could possibly be to low, it is mounted as high as it can go. The top of the bracket is flush with the top of the transom, the motor is all the way up on the bracket
Joe, I have always been under the impression the raising a motor created less drag, put the prop in cleaner water, increased rpm's and improved top end, I guess I was wrong about that
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| Joe Kriz |
Posted on 09/07/09 - 5:55 PM
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Paul,
In theory you are probably correct.
The bottom line is where the cavitation plate is when you are running the boat at speed and all trimmed out with a normal load.
The cavitation plate should be just slightly above the water. Not under the water.
Have someone drive the boat for you and take a look at the cavitation plate and let us know if you can see it or not...
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| Paul Graham |
Posted on 09/07/09 - 7:10 PM
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I don't know how much if any, water is in the hull, but the first time I launched the boat I paid special attention to how she sat in the water, I thought it sat perfectly considering the weight of the motor and jack plate. I would say the the bottom edge of the hull just below the lettering was about an inch below the water. I am going to launch one evening this week and will take pictures of how she sits and make note of where the cavitation plate is at planning speed. I also did check throttle travel yesterday, when at full throttle the throttle cam is in full open position, so I pretty sure I'm getting full throttle. The last 3 pictures I just posted on my personal page will show how the motor is mounted. You can also compare how the older hull was rigged in comparison to the newer hull, only the last 3 photo's are of the newer hull, thanks gentlemen, I'm learning a bunch here.
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| Tom W Clark |
Posted on 09/07/09 - 7:26 PM
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Paul,
Motor mounting height looks good. Boat looks good. I remain perplexed as to why there would be so great an RPM drop when you switched this motor to the newer hull.
Something is funky here. Are you sure the motor is firing on all four cylinders? Have you made sure the spark plug boots are all seated?
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| Paul Graham |
Posted on 09/07/09 - 8:05 PM
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It runs a little rough at idle occasionally but otherwise seams to run fine, I'll check the spark plug boots tomorrow. Wouldn't it run like crap if were firing on only 3 of 4 cylinders?
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| Tom W Clark |
Posted on 09/08/09 - 7:53 AM
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No, not necessarily.
My own boat lost three cylinders on one of its V-6 motors. That motor still ran and idled, but did not have much power.
I had the motor repaired (replaced defective switch box) and when I got the boat back, I noticed that the other motor needed a little more throttle to maintain the same RPM. It otherwise sounded and idled just fine.
It turned out that one of the spark plug wires was missing after I got it back for the shop. Boy, was my mechanic embarrassed!
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| Joe Kriz |
Posted on 09/08/09 - 11:11 AM
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Paul,
From the photos it certainly looks like the engine is high enough...
Hopefully not too high but only running it will tell you that...
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| Paul Graham |
Posted on 09/29/09 - 7:56 AM
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I have gone thru the carbs and made sure that I am running on all cylinders. I picked up a 15" x 13.5" pro and only got 5300 rpm's. The Honda mechanics tell me I should be running 6000 rpm's, so I went back to the prop shop and have a 13" x 13" prop coming. I was wondering, does anyone else on this site have a 75hp Honda on a smirked montauk. I would like to know what prop they are running and what RPM's they are turning, thanks
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| Paul Graham |
Posted on 10/01/09 - 6:38 PM
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Tom, I got the hotdogs, thanks, I owe you. I finally took a photo of the boat launched. Can I get opinions on weather she looks water logged, it's the last photo on my personal page, thanks
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| Tom W Clark |
Posted on 10/01/09 - 7:20 PM
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Paul,
Your personal page does not show any photo of your current boat floating in the water.
The last photo there is of the bracket and it looks like it was taken in your driveway at night.
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| Paul Graham |
Posted on 10/01/09 - 7:24 PM
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I think I got it posted now, thanks
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| Tom W Clark |
Posted on 10/01/09 - 7:30 PM
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No, that hull does NOT look waterlogged.
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| Paul Graham |
Posted on 10/02/09 - 1:28 PM
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Thanks Tom
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| Tom W Clark |
Posted on 10/02/09 - 1:43 PM
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Are you sure the throttle is reaching its fully open position at WOT? You've checked your cables?
There is no way that boat should require a 13" pitch prop.
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