Is my motor/prop too low?

[JRDIII]

I recently bought a 1962 Boston Whaler 13-footer with a 1992 Johnson 40 hp. The boat is in decent shape, and I plan to work gradually on restoring it. In the meantime, I just want to enjoy it. The motor runs pretty well, although it idles rough and will sometimes cut off when going from idle into gear. Probably needs a thorough carb clean and/or a new power pack. But once you get it going it runs great. It gets up on plane quickly and the motor is very smooth at full throttle.
My question is about the prop placement, which seems to me like it may be too low. Out of the water with the motor down, the anti-cavitation plate is just about even with the bottom of the hull, which is where it should be. But when the boat is at rest on the water, almost the entire lower unit is submerged. On plane, the anti-cavitation plate seems to be under the surface, although it's hard for me to safely tell. At full throttle on smooth water I am maxing out at 27-29 mph according to GPS. From everything I've read, this boat with a 40 hp ought to be in the low- to mid-30s. The motor is mounted in the top holes of the motor bracket, and the bracket rests on the transom. Should I consider raising it to the next set of holes down, or even two sets of holes down? I would have to drill new holes for the bottom bolts, which is not something I really want to do if I can avoid it.

I have some video from last weekend where you can get an idea of the motor position. FYI, the prop appears to be original and is 17 pitch. Can any of you experts get enough of a view from this video to suggest whether I should make an adjustment or just leave it as is?

http://youtu.be/1fpJDIun2Yw

Here is a picture of how the motor is mounted (sorry, can't seem to get photo to rotate):

 

[Tim Frank]

Re: Is my motor/prop too low?

You need to know what RPM you are turning if you are worried about correct prop....that will obviously also determine max speed.

 

[emoney]

Re: Is my motor/prop too low?

If you cav plate is flush with the bottom of the hull, that sounds right. Might be better to take a picture of this area and post that. As far as the video goes, it doesn't look like it's too low from the wake, which seems about normal. Your boat looks to be stern heavy which is probably why it seems so low from the helm. You could move the batteries and fuel tank forward if it's a real concern.

As far as "top speed" goes, don't forget a lot of those numbers tend to be exaggerated. Cleaning the carbs, of course, will increase performance so until I did all the needed maintenance I wouldn't worry too much.

 

[jleiii]

Re: Is my motor/prop too low?

JRDIII,
My son and I just put a 1970 BW 13 sport back in service with a (new to the boat) 1960 Johnson 40 HP. The first thing I noticed was that the angle of the transom is WAY to much for the engine to sit properly. I added a 1.5" tapered wedge between the transom and motor bracket and that was about perfect. We've only had it in the water the last 3 weekends, and have not had a chance to really fine tune the angle (4 stops in mount), but we clocked it at 25 knts (about 28 mph) this past weekend. On our return trip it started acting up and I found that the high speed mixture screw had closed itself to the stop. I adjusted it to max open and the speed was increasing the whole time. Next time out we will reset and adjust the screw properly with GPS in hand. The boat last had a 25 hp Evinrude on it and my father reported 25 knts with that, so I expect better with the 40. The engine angle should also have a measurable impact on speed.

Our battery is ahead of the amidship seat, gas tank (12 gal) under the back seat. Both made a big difference in the way the boat sits and rides in the water. Being an older motor it's probably heavier than yours (136 lbs), and that would make it sit lower. My anti-cavitation plate is inline with the bottom of the hull.

It's tough to be sure from the video, but it looks like it's riding with the motor leaning back a tad. Adjust it out if you can and see if that makes a difference. Prop tuning is a whole other magic process! Been there with my inboards, and not cheap. I found a New-old-stock prop on ebay for mine, and I'm not impressed with the quality from the factory (Michigan Prop). Has to better than the the original regardless.

John

 

[JRDIII]

Re: Is my motor/prop too low?

Thanks for the input. I agree that the angle of the transom makes it seem like the motor tilts too far inward. But once the boat's up on plane, that angle becomes just about perpendicular to the water. My understanding is that you want the prop to be perfectly perpendicular when you are on plane. Once you get your motor working properly, I'd be interested to know if the wedge makes any difference in your speed. I'd also like to see a photo of it if you can attach one. As I mentioned, my top speed is consistently 27-29 mph, which is the same as you measured so far.

I also agree with the comments that the stern does seem to sit low. I may try experimenting with moving the battery and/or gas tank, but the truth is, I really like them where they are right now. I use the boat for fishing 90-percent of the time, so the fewer items in front the better. I even remove the forward bench seat when I'm by myself.

The idea of trying to re-prop the motor doesn't appeal to me right now. From everything I've read on this site and others, re-propping sounds like an expensive undertaking that only occasionally generates improved results. I can't believe how many posts I've read from people who spent a lot of money on a new prop and the results were either worse or only marginally better.

As I said, my only real concern is whether the motor/prop are too low, and whether raising the motor will improve my top speed. If it's not going to make much of a difference, then I am content to leave it where it is. I can live with 27-29 mph. But if I can gain 5-7 mph by moving it up one or two holes, then I might just try it.

 

[Tim Frank]

Re: Is my motor/prop too low?

You may be missing the point of re-propping.
It is cheap insurance for your motor's health.

 

[jleiii]

Re: Is my motor/prop too low?

You may be missing the point of re-propping.
It is cheap insurance for your motor's health.

On a 13 foot whaler with 40 HP?? Unless it's the wrong prop or damaged this is highly unlikely. Add a dozen feet and a hundred HP, then it maybe worth the investment to tune the prop. There is nothing 'cheap' about this.

RDFIII, you can find a pic of mine in this post:
BW 13

 

[JB]

Re: Is my motor/prop too low?

Though the BW 13s are rated for 40HP they perform and handle best with 25. The weight of a 40 really messes up dynamics of planing and handling.

 

[Tim Frank]

Re: Is my motor/prop too low?

On a 13 foot whaler with 40 HP?? Unless it's the wrong prop or damaged this is highly unlikely. Add a dozen feet and a hundred HP, then it maybe worth the investment to tune the prop. There is nothing 'cheap' about this.

I guess you are missing the point as well.
As far as cost....it could cost you as little as nothing....if you don't need to take any action.

We have a 1962 vintage boat....a 1992 vintage motor....and complaints of lack of performance, although the motor "runs good"..
No idea of WOT RPM....and no comment on prop or its condition.
No idea what the 1992 motor history is/was....did it come off a 'toon....or something large?

etc. etc.

OP should verify that he is getting acceptable service from his current prop....or at least determine what parameters he has.

 

[JRDIII]

Re: Is my motor/prop too low?

I guess you are missing the point as well.
As far as cost....it could cost you as little as nothing....if you don't need to take any action.

We have a 1962 vintage boat....a 1992 vintage motor....and complaints of lack of performance, although the motor "runs good"..
No idea of WOT RPM....and no comment on prop or its condition.
No idea what the 1992 motor history is/was....did it come off a 'toon....or something large?

etc. etc.

OP should verify that he is getting acceptable service from his current prop....or at least determine what parameters he has.

To be clear, I have not really complained about a lack of performance. In fact, as I mentioned I am pretty happy with 27-29 mph as it suits me OK. However, if I can gain 5-7 mph by raising the motor a notch or two, I might be interested in doing that. I have posted a video and a photo so that the experts here can get some idea of what my setup is like, and whether they think it would be worth it to try to move the motor. The prop is in good shape, has good grip, gets the boat on plane very quickly (as you can see in the video), and I have no interest in changing it at this time. Thanks!

 

[emoney]

Re: Is my motor/prop too low?

There is one thing I can't see in the video or the picture and that's your trim pin. In looking at the video for the 2nd time, it looks like your motor is trimmed in a little too far because your spray starts toward the first 1/3 of the boat. Ideally, you want the spray/wake to begin at about or beyond the 1/2 way point for optimal performance. You didn't mention having power trim/tilt so I'm assuming it's manual.
If it is manual, the "trim pin" is a bar that extends from one side to the other (or between each bracket) that stops the motor from going down any further. Normally, there's a few options on where to put this pin/bar (4-5 at least). If you have this type setup (a good picture from the side but zoomed out alittle would help) can you tell us where that pin/bar is now?

 

[emoney]

Re: Is my motor/prop too low?

In this video, you can see what I'm talking about as it applies to the wake/spray;
http://www.youtube.com/watch?v=ljJ2nB3sA60&feature=related
It's towards the end of the video, btw.

 

[JRDIII]

Re: Is my motor/prop too low?

In this video, you can see what I'm talking about as it applies to the wake/spray;
http://www.youtube.com/watch?v=ljJ2nB3sA60&feature=related
It's towards the end of the video, btw.

Yes, I should have mentioned the trim pin is out as far as it will go. The transom has a pretty steep angle, which makes the motor seem trimmed in too far even with the trim pin set on the outer holes. Someone up above with a similar setup mentioned they put a tapered wedge in between the transom and the motor bracket to push it out further. I would like to see a photo of what that looks like and what material he used.

 

[Tim Frank]

Re: Is my motor/prop too low?

Some good info here, with the prop change, or deliberately-determined-no-need to change, criteria better defined than I perhaps have done.

=> http://www.deltaprop.com/linkso.asp

I had missed the video, but after seeing it I am also in the too-far-forward-tilt crowd. Have you tried raising the tilt one notch?
I'd be hesitant to change the mounted height of the motor....presumably that was done correctly when the motor was installed....and your description and pics don't seem out of line.




Q. Why change propellers?

A. The stock propeller with which most outboards are equipped is a compromise. Since it has fixed diameter and pitch, it is really limited in its use and it does not provide satisfactory performance for all the combinations of hulls and loads that will be encountered once it is installed. One important fact to note is that the propeller moves the boat through the water at a specific engine rpm, and h.p. is directly related to the developed rpm. The engine cover is marked with a certain h.p. rating but in most instances the full benefit of the possible h.p. is never realized. Along with the h.p. rating equal emphasis should be placed on the rpm at which the rated h.p. is developed. This, of course, is where the propeller comes into the picture. Outboard engines are designed to be run at peak rpm for full efficiency. Excessive rpm with its increased friction and wear is obviously harmful. It is equally harmful to run the engine so overloaded that it cannot achieve its rated rpm since this results in excessive carbon build-up in the cylinder with subsequent problems of poor fuel economy, pre-ignition, frequent spark plug failure, scoring of the cylinder walls and even burned pistons.

 

[emoney]

Re: Is my motor/prop too low?

I'd imagine that the '63 boat wasn't prepared for the '92 bracket, lol. I saw the "angle bracket" and I'd think that would end up being some pretty serious engineering to get right, unless it's something they sell exclusively for this boat and this issue. Otherwise, you flirt with changing the engineering dynamics of the original design and for me, at least, that's a territory I don't like to tread into. A 13' doesn't need a lot of bow rise as it is, so since we're not talking about going from night to day anyway, that's probably one of those "leave it alone" areas. At least to me, anyway.

If you look at the picture of the boat that jeiii posted, you can see that the 1960 outboard had a bracket that pretty much compensated for angle. Even without a bracket,that outboard probably hangs quite a bit more perpendicular than yours.

Congrats on the boat, btw, if I didn't already say that. Looks like you and the 1st mate were having a blast on it in the video. You'll find lots of like-minded folks here and some very helpful people as well.

 

[JRDIII]

Re: Is my motor/prop too low?

The tilt is a manual hydraulic tilt-assist system, and as far as I can tell the only options are either all the way up or all the way down. There is no in-between setting. The trim pin is set in the outer holes. Is there another tilt/trim setting I am missing? I'll take some photos tonight and post them.

 

[JRDIII]

Re: Is my motor/prop too low?

OK, here are some more photos of the motor. I figured out that you can stop the motor from going all the down by locking the tilt-assist lever up before it hits bottom. But, I don't think I would want to run the boat with the motor locked like that. When I try to lift it up from that position, it still has about six inches of play before resistance kicks in, but if I were to hit a rock I think there could be a major problem. My understanding is that when you run the boat, the tilt assist lever should be all the way released. Please let me know if that is not correct.

The first picture shows the motor all the way up. The second shows it locked in a nearly perpendicular position. The third shows it all the way released, which is how I have been running it. And the last two photos show the trim pin in the outer holes. And thanks for all the input so far!

 

[nwcove]

Re: Is my motor/prop too low?

jmo, but it looks as tho...somewhere between pic #2 and pic#3 would be the right trim angle. in pic # 3 it appears to be tilted in way to far for best performance.

 

[JRDIII]

Re: Is my motor/prop too low?

FYI, for Tim Frank, here is a photo of the prop. It seems to me to be in pretty decent shape, but I'm open to any suggestions if you can tell anything from this photo. It is a 17 pitch and as I mentioned it gets the boat on plane very quickly and runs smoothly. I don't own a tachometer, so I haven't measured RPMs. If you really think I should, I may replace it down the road, but for right now it's not my highest priority.

 

[JRDIII]

Re: Is my motor/prop too low?

jmo, but it looks as tho...somewhere between pic #2 and pic#3 would be the right trim angle. in pic # 3 it appears to be tilted in way to far for best performance.

That seems to be the consensus, but I'm not sure what my options are unless I lock the motor in the correct position, or add a wedge/shim under the motor bracket. It seems to me that there ought to be a simpler solution, unless this boat and motor just weren't made for each other.