Performance of two boats different shafts..

[kaboominator]

Hi, was just looking to get some info on performance results on two different boats with two different motors/shaft lengths. My friend and I each have acquired new boats/motors. Both with unique performance issues. At time of post i havent yet measured the shaft/transom heights but will be asap. My friend has a 1999 25 hp 2stroke johnson on a new 17 foot sylvan jon boat. From my perspective the motor is a longshaft when he shoudl have a short shaft. with a single person in it at WOT the bow of the boat is being forced down into the water. Not planeing well at all. And that is with the tilt on the highest setting. the lower you go the worse it is. Basically when we put three people in the boat we all need to sit at back bench and then she will go a bit faster. but when the boat is sitting in the water witht the motor down it looks to me to be sitting way to low in the water and the angle is of such that i can understand when under force how it wants to force the bow forward. And mine, which i beleive is simple, the caviataion plate, when at WOT sits just above the bottom of the boat which causes water splashback. I beleive i should have a long shaft or do some modification to the transom. Any points on either of these isssues?

 

[rickdb1boat]

Re: Performance of two boats different shafts..

Start by looking to see where the cavitation plate is in relation to the bottom of the hull. It should be even or slightly above.

 

[deejaycee_2000]

Re: Performance of two boats different shafts..

why don't you and your friend swop motors and see how it runs, if you need a long shaft and he the short ... try it ...... otherwise rickdb1boat's diagram is spot on .....

 

[Chris1956]

Re: Performance of two boats different shafts..

Kaboom, Your description seems to be a symptom of the motor shaft being too long. Does the motor clamp bracket have a mechanism to raise up the motor? If not, or in conjunction, your friend could put a piece of wood on top of the transom to raise the motor up a bit. If the motor has transom bolts, be sure to install them to keep the motor on the boat.

 

[kaboominator]

Re: Performance of two boats different shafts..

Hey, thanks fella's for the replies. We will not be ablet to switch as i have a 14 foot aluminum with 15 hp merc and he has this gigantic 25 hp johnson. It seems so big for only 25. Anyway i will be looking into all your suggestions this evening and i will get back. Thank you for your suggestions!

 

[kaboominator]

Re: Performance of two boats different shafts..

Well the transom on the 17 foot Sylvan jon boat is 20 inches and the 25 hp Johnson is a 20 inch long shaft so it should be fine. The guy just seems to think it is a poorly designed boat! I think there must be something to do. That is plenty power to push that boat. I'm not sure if anyone is familiar with the Sylvan design but the transom has quite an angle on it. The tilt cotter pin is on lowest setting and if you go any higher it just plows harder. not sure of what type of remedy is possible here. All i know is that another buddy of ours has 16 foot Lowe jon boat with 25 hp merc SS and he flies on the water. Planes perfect and skims along without pushing any water at all. Not sure of what can be done here. The motor is matched to the boat! any other ideas or is it just a poor design that should be accepted. It has the slight V front that flows into flat bottom.

 

[Texasmark]

Re: Performance of two boats different shafts..

Ok, how about this. What is the slope of the transom?

With the boat sitting on the trailer, and the engine at the highest tilt pin position, stand to the side of it and tell us what is the (tilted) position of your motor as compared to Rick's pic.

In his pic, the anti vent plate is parallel to the hull. If your Sylvan has a high sloping transom, like my al boat does, you may not have enough range on your tilt pin to push the motor out far enough to get the bow up. If so, your engine would be sloped back (power head to the rear rather than powerhead forward).

For what it's worth, due to some jabbering on here lately, I have surfed sites like www.old-omc.de and noticed that the smaller motors anti vent plate sits below the bottom-flush line, like an inch or so, not like the pic and words mentioned above. (Thats where you want it if you have a high hp or high perf rig.)

Possibly the reason is that a lot of smaller boats have a predominant keel (especially alum) and with a "non high performance" alum prop, that most engines come with and use in the lower hp's, you would have a lot of "blow out" problems, especially in turns.

I never ran a 20" on a 15" but I personally doubt that a long shaft engine would cause a nose down problem.

Since the boat is propelled with the propeller, the force is at the prop. The farther the prop is from the boat, the longer the arm (moment of inertia arm....tech term). Kind of like ft-lbs of force. Prop has a fixed thrust and the longer the arm, the more the force on the boat; just like a torque wrench.

With adequate tilt (out) the lower the propeller, the longer the thrust arm, and the higher the bow should be out of the water.

What would be affected, would be boat speed (lower) due to the added drag of the extra 5" of lower unit, but not that much, I'd think, because from the water attack angle, the lower unit is pretty thin and he's not going to the races with that rig so he's not doing that many mph to make that much difference.

Check the tilt and come back.

My 2c but I think it has merit.

Mark

 

[kaboominator]

Re: Performance of two boats different shafts..

Excellent explanation Texas. I will take a look tonight and see what the angle is on the highest pin position and verify the plate location in realation to the bottom flush line and get back to you. Thank you for your reply!

 

[Texasmark]

Re: Performance of two boats different shafts..

Thanks, waiting to hear from you.

Mark

 

[steelespike]

Re: Performance of two boats different shafts..

It appears the 25 is the right length.So it is probably a trim issue.If the transom has a lot of rake you can buy or make wedges to go between the clamp and the transom to put the motor more perpendicular.
Check the bottom of the boat for a hook. You can use a straight edge about 4 ft to do this.

 

[kaboominator]

Re: Performance of two boats different shafts..

okay on the highest pin position the angle is still slightly off perpendicular to the transom, so im assuming a wedge would be the way to go. the transom has approx and angle of 25 degrees. So we will try and construct a wedge to see if that helps. Will get back when we try that. thanks again.

 

[Texasmark]

Re: Performance of two boats different shafts..

Kicking this up........I thought so. 25 degrees is like my current boat......way too high...........but it's good to help to dump the return wake when you shut her down and the boat sinks and you don't want backwash to engulf your boat.

You are on the right track man, hang in there.

Mark

 

[Texasmark]

Re: Performance of two boats different shafts..

Hey, Kaboominator, don't drop us on this. I participated in this and I am keenly interested in the solution.

Mark

 

[rickdb1boat]

Re: Performance of two boats different shafts..

^^^

 

[kaboominator]

Re: Performance of two boats different shafts..

Thanks guys..We are making the wedge this weekend and will be trying to finally fix this beast. I will keep you posted. Thanks for the interest.

 

[Texasmark]

Re: Performance of two boats different shafts..

Kicking it up for the answer. 8)

Mark

 

[kaboominator]

Re: Performance of two boats different shafts..

Hi there. Well we poked around some more this past weekend but didnt really come to a solution. I didnt describe one of my prior posts acurately. Although when on highest pin position the motor still is off perpendicular, but he has been running it on the lowest postion with the best performance. Apparently the higher you place the pin the more the bow is forcing down in the water. So...the wedge is still going to be tried...I almost want to make it yself and try it. Hes been busy building agarage so didnt have much time for pokin around...but ill get on it and keep ya posted...im as curious as you.....

 

[Texasmark]

Re: Performance of two boats different shafts..

Man you just blew me away. :%

Highest pin setting pushes the prop farthest out from the transom which puts max up pressure on the bow. We have our wires crossed here somewhere. Plowing (nose down) is caused by it being tucked in too close to the boat (lowest pin setting).

Come back.

Mark

 

[kaboominator]

Re: Performance of two boats different shafts..

Oh man im so sorry:/ I totally understand what you are saying and i understand the logic. That is why i am very confused. Having it on the lowest setting towards the transom currently is the only option becasue the higher you go the more plowing there is. I know this makes no sense. The only thing i can thin is that the boat is so long and flat that has that much more trouble getting on plane. i dont know...baffled..its a Sylvan Jon Boat 17 feet. Model 1750 MVX. here is a link with the details on it. http://www.quinnsmarina.ca/sylvan-04/jons_mvx.htm
Im super confused...If ya got any more tricks in your bag im here Mark...thanks again for your time.. Kyle

 

[steelespike]

Re: Performance of two boats different shafts..

I think I would try the wedges.I think you still need more out trim.If the bottom of the boat is in good shape there is no reason it shouldn't plane properly.
I think some Jons plane really flat but it shouldn't plow.