Hello Mr D Hadley

[Texasmark]

Morning D. Just came from the prop section and you were talking to a guy about engine height on a smaller hp outboard.

I have observed many engine heights in the sales brochure pictures in the old-omc de/ site and they (anti-vent plate) all seem to be roughly an inch below the hull.

Beins you were/are an OMC dealer, could/would you shed some light on this sir?

I was thinking that maybe the lower hp engines, or the low gear ratio's they ran back then, or the design of the props required the plate to be submerged during operation.

I think I can remember personal situations where I may have seen it below on my stuff. I also remember putting a piece of 1x2 wood under the clamp bracket on a '75 70 hp rude I had to bring the plate up higher.....rig ran much faster. Would have tried another one but there was something about the rigidity of the installation that caused me not to...don't remember.

Anyway, on current rigs of the higher hp and H.P. props and all being above is a no brainer....you guys and others prove it all the time. I run mine above.

I'm not sure, but I seem to recall that once upon a time in my life I measured the distance from the clamp bracket seating surface to the plate and it was 16" not 15.....might have been on my '58 (I think) 18 hp rude.

Course back then we never heard of blowout and cavitation/ventilation and all that stuff. The props just dug in and pushed the boats.....course that was at 20-30 mph too. d

Thanks,

Mark

Thanks,

Mark

 

[Texasmark]

Re: Hello Mr D Hadley

Hey D, what a coincidence. Check out this thread 2 up the list from this one and you can see exactly what I am talking about.
Interesting Hydrofoil True Batwing.

Mark

 

[Dhadley]

Re: Hello Mr D Hadley

You guys hit the nail on the head in the other thread. "Back in the day" the water pick ups weren't the greatest (efficient) nor were the props. The transoms on small aluminum boats were all over the palce. Some 15", some 18" and some 20". I remember one manufacturer having an 17" or 18" transom saying you could run a 15 or a 20" motor.

And gas was a tiny bit cheaper so I guess no one cared much about efficiency. But I also had small boards under the motor.

 

[deejaycee_2000]

Re: Hello Mr D Hadley

Hey Mark just for interest sake .... over the years I have done test after test on the height of my full inflatables ... and the fastest, best, most efficient way I fine tuned the height is when the middle of the prop is exactly in line with the middle of the highjacker's air intake valves (the highjacker is the small pontoon tube that runs in the channel at the bottom, left and right, to the front of the boat) .... if you have a close look at the pic below you can see that the anti-cavitation plate is below the bottom of the boat but the small fin above the anti-cavitation plate is inline with the bottom .....

 

[Texasmark]

Re: Hello Mr D Hadley

Whooooa. Back the truck up Dee. I don't know how your boat runs when it's not up in the air. d d d

I see the second pic and understand what you say, but I'd bet that if you raised the engine such that the vent plate was in line with what you call "the bottom", the boat would move alright, but every time you goosed it all you'd get would be a rooster tail and no/little forward motion.

Where is the "bottom" of the boat when you are running in calm water straight out @ WOT? How much of the prop is in the water?

Gonna betcha 1/2 to 3/4 of the prop is all that's in the water and the only thing touching the "bottom" is spray off the sponsons. 8) Otherwise there would be no reason for the engine to be mounted so low.

So I can't use your application as a reference.....but thanks anyway.

How's the dislocated shoulder and cracked ribs doing or was it your back that you knocked out?

That's one heck of an inflatable you have there. Bet a Zodiac doesn't look like that underneath!

Interesting blade design on your prop too. Who does those for you?

Mark

 

[deejaycee_2000]

Re: Hello Mr D Hadley

Hehehe .... what I call the bottom is the middle flat part, but that part is never in the water when the boat is running, so if I raise the motor so the vent plate is in line with it as per normal on all other boats, it gets nowhere, the motor just cavitates a hell of alot ..... but you are right only about 3/4 of the prop is in the water and the pontoons just touch the water every now and then ..... I don't think any boat's math on engine height can be based on my boat for a reference hahahaha ..... just thought you might enjoy it, so I gave you something to think about for interest sake, I know you like to think technical .. hehehe .... health wise ... I'm in tip top condition now and all bones are back to normal and practising for the end of year 5 day race .... you might see me on ESPN hahaha .... the props I get custom made, I have a couple of friends that specialise in stainless steel and engineering, made into a 16 pitch stainless steel racing prop, look alot different from a normal prop very sharp, cuts like a hot knife through butter, the cup has been perfected over the years to minimise cavitation and add to top end speed ..... here is a couple of pics when running in flat water WOT so you can see what part is in the water .....

 

[Texasmark]

Re: Hello Mr D Hadley

Very nice pics and they support my assumption.

You never told us just how fast you are going when you are racing. The engines look like OMC 3 cyl and that maxed out stock at 75 hp and you probably added 25 hp or so with your souping up. So you are running roughly 100 hp on essentially no water resistance......gotta be doing 65 or 70 anyway.

Mark

 

[deejaycee_2000]

Re: Hello Mr D Hadley

..... hehehe .... no we race with 3 cyl 50hp motors (I only race with Yamaha), souped up to about 78hp .... the 75hp yummy I was talking about in other post long ago, was just an experiment motor I tested as we had to custom add a tiller etc. and modifying it to the max to about 112hp ..... the fastest my souped up 50hp runs at WOT in a race on flat water with the wind is about 85mph on gps ......

 

[Texasmark]

Re: Hello Mr D Hadley

Man oh man. Moving right along......not hard to see why with the rigs you guys are running.

Thanks for the info and great shots,

Mark

 

[deejaycee_2000]

Re: Hello Mr D Hadley

Hehehehe ... always fun chatting to you .... if you ever swing past South Africa, please let me know, i'll take you for a spin ....

 

[Texasmark]

Re: Hello Mr D Hadley

Oh no you won't. 8) 8) 8)

But I'll have a beer with you.

Mark

 

[deejaycee_2000]

Re: Hello Mr D Hadley

hehehehe, ok cool ....