Motor Height?

[cleve]

Which part of the boat bottom should my AV plate be even with? Bottom of transom or step?
'93 Johnson 90hp on a 17' Javelin 360A

 

[Sea Rider]

It's relative. Ideal would be for water flow at spped to skim right under small upper splash plate, just where the sharp edge is to cut flow nicely. Will need to have someone follow you or pull head out tramsom to visually determine where is flow passing by through lower leg. Raise or lower engine to achieve that condition.

Happy Boating

 

[Faztbullet]

Where its at now is general purpose all around use. If all you do is fish and want to go fast it can be raised 1 hole and lose the aluminum prop. On hulls with a pad the best stating point is to split the water intakes with bottom of hull

 

[Texasmark]

Get 2 more shots: 1 from behind so we can see the hull shape (engine out of the way), in particular the hull at the Keel location. 2 from the side, 90 degrees out engine at about 20 degrees tilt up from vertical, camera positioned even with keel so the eye can scan from the bottom of the hull out to and including the AV plate location.

While doing that how about some giving us some stats on your current prop including dia and pitch, cupped or not, ports or not, and performance numbers where you do the most running: boat speed and rpms at WOT, trim position.

 

[cleve]

I'm asking about the motor height because I've never owned a hull with a pad.
The hull is stern heavy, partly by design. But, it's an old boat that probably has some waterlog, is slightly underpowered, and the motor is old with lower compression (100-105 psi but runs great).

At WOT, I can trim it out forever before the prop even thinks about venting, and the whale tail is roughly even with the water surface at WOT, slightly above at times. The highest RPMs I ever got was 5000 (2 people average load) and prop is 13.25x17 AL Michigan Wheel with no dings (pretty good shape). Fastest speed I've got was 39.7 mph (just me), but average is 35-38 mph and it's kind of a dog on hole shot.

I thought raising 1-2 holes may help a little, but realize I'm not working with much.

 

[jimmbo]

Well with out seeing the hull from behind, it is difficult to see how big the pad is, or if there is one. As for being even with the bottom of the hull/keel or the bottom of the stepback, where does the motor sit currently, Looking at the pic it is mounted down as low as can be. As mentioned you might be able to run it one hole higher with the Aluminum prop, but any higher will usually require a higher performance prop with more rake and cup. As mentioned, more pics that are sharp and properly lit and exposed will be of great help

FYI raising the engine gives about 3/4 - 1 1/2 mph speed increase per inch upwards, but that is at speeds above 55mph. It also reduces the amount you can raise the bow trimming upward/outward, and it does result in some ventilation of acceleration(rake and cupping in the prop help here)

 

[JimS123]

We really can't help you. You need to figure it out for yourself. All I can say is that in most cases performance can be improved by moving the motor up.

I have run different positions on different boats, and different positions with different motors on the same boat. 3-blade, 4-blade, SS or aluminum - all factors to consider.

I try all positions, and then generate performance curve data at each position. If you try another prop, do it over again. Its all trial and error, and part of the fun of owning a boat.

 

[Chris1956]

Gee, I didn't see a Whale tail in the photo. You will probably want to test with and w/o it to make sure it isn't dragging.

Since you can trim out a lot before ventilation, I would raise the motor 1 hole and recheck speed and max RPM.

Then repeat until speed and max RPM drop off, due to ventilation. Setting the motor height for max RPM can cause driveability issues. I will usually drop the motor down 1 hole from max performance so she is more driveable.

A prop (al or SS) with more rake will help with performance. I like 3 blade Turning Point Hustler in AL, and thee are many choices in SS, Rapture being one.

 

[Sea Rider]

Forget all about AV plate must be even with boat's rear hull, each OB has its own height parameters that usually varies from 0.5 to 1.5 " and that's on dried installations, which doesn't say much, when combo is on plane it's a different wet story. A short cut if wanting to follow it to the letter..



Go for a wot spin as loaded on flat calm water cond, with deck weight evenly distributed, once at plane wihile OB sitts at 90 deg, visually check, have some one follow you closely at rear, determine at which lower leg height is water flow passing at speed. Ideal is for flow (>) to skim between narrow edge and dotted yellow line (upper water splash plate) If passing right above it will have water splashes out, upwards transom which will slow combo a bit due to extra unwanted drag going on there.

That OB being old has very limited height between both lower leg plates to play with, so this setting must be dialed very precisely. If flow passes near AV plate (red line) OB will experience prop aeration on choppy water cond, tight close turns, only good for straight water courses on calm water con.

Report at which lower leg height is flow passing at speed, can go from there fine tuning OB height.

Happy Boating

 

[Faztbullet]

The OP needs to google outboard mounting height and read for himself as your info is incorrect for his setup. Its ok for smaller motors ......

 

[cleve]

Gee, I didn't see a Whale tail in the photo. You will probably want to test with and w/o it to make sure it isn't dragging.

Since you can trim out a lot before ventilation, I would raise the motor 1 hole and recheck speed and max RPM.

Then repeat until speed and max RPM drop off, due to ventilation. Setting the motor height for max RPM can cause driveability issues. I will usually drop the motor down 1 hole from max performance so she is more driveable.

A prop (al or SS) with more rake will help with performance. I like 3 blade Turning Point Hustler in AL, and thee are many choices in SS, Rapture being one.

Correct, I had taken the whale tail off when I took this picture last Summer to try the boat without it, but put it back on.
WOT handling wasn't as good and hole shot was even worse. Also, there was no difference in top speed.

 

[Texasmark]

I don't think you are going to get there from here: Low compression (book says anything less than 120 can cause problems) means lack of what you think to be your hp in the first place, which is low. Poor hole shots even with the WT means low power/too much pitch verified by your max 5000 rpm at WOT setting, heavy stern as you mentioned but batteries, fuel, and livewell can aid in that poor shot.

Alum prop means no cupping or progressive pitch (unless a Turning Point Brand, Hustler) nor ports to assist in getting a faster hole shot; 90 hp out of the factory on that boat is marginal. Whale tail a hindrance to higher speed. If you saw all of it your prop would be partially out of the water and would be ventilating causing your rpms to rise and speed to fall off, otherwise unnecessary drag.

Usually when you drop pitch to bring rpms up and improve hole shot, top end suffers. Raising engine to attempt to get better performance, with an alum prop can/will result in ventilation on hole shot possibly....depending on how you execute it, turns for sure.... the tighter the worse, especially with that hull, and running in a goodly amount of chop will ventilate.

No logical sequence here to my answer, just brainstorming as I think of things.

I agree with Jim. Sorry!

 

[Sea Rider]

The OP needs to google outboard mounting height and read for himself as your info is incorrect for his setup. Its ok for smaller motors ......

Google won't help a thing, you have to make water tests on your own, should know that by now. Theory most of the time doesn't match practice.

The premature conjecture to which you have arrived without having experienced it personally before do not assures anything positive to the OP.

Happy Boating

 

[Faztbullet]

The premature conjecture to which you have arrived without having experienced it personally before do not assures anything positive to the OP.

I have been setting up motors and racing them for over 35 years. I have been over 120mph on water and I know how to advise someone how to set up their motor and get them pretty dang close. And yes Google works....

 

[ahicks]

The OP's idea of increased low end by raising the engine is not likely going to work out/end well. Any increases seen (assuming there is an increase) is way more likely to be noticed at speed - if you can get it to speed without venting/cavitating.

 

[Sea Rider]

The issue here is that we're talking about recreational/leisure/family boating not speed racing boating which are two diametrically opposed scenarios. Nobody doubts your abilities, experience, expertise doing that type of boating.

Recapitulating : OP needs to determine what's going at back transom at speed with a visual check if in pursuit to get the max out of hat combo, If not, will continue boating at Limbowaterland...

In reality this is what's happening : when combo is on plane at speed, water passing under middle hull compacts, flatens out, the shape of this hardened water flow exiting rear middle hull will take the shape of it if happens to be flat, round, shallow V or deep V.

Need to match that shape to lower leg to achieve the least drag and top prop thrust, that happens right under small water splash plate and front sharp edge in all motors. Newer ones have larger distance between lower plates, Jurassic ones way less to play with. Why is it so difficult to digest this tech concept ?

You have me typecast in that I only work with portable/medium motors, now messing with up tp 150 HP ones and applying this same transom/engine height concept employed on my previous posts with same excellent results.

Happy Boating