Should i raise to 41/4 ptp

[Faztbullet]

This is from BRP/OMC prop guide book.. cant argue with the manufacture as they know best....


When you have a selection of propellers ready for testing:

1. Use an accurate tachometer to measure RPM and an accurate speedometer to measure

boat speed.

2. Testing should be performed with the typical load—number of people, gear, water in live

wells, etc.

3. Make sure that every test is with an identical setup.

4. Test each propeller at wide open throttle (WOT).

5. Engines should be tested at their optimum trim angle. This is the highest trim position the

engine can be run without excessive ventilation, either in a straight line or in turns.

6. If the RPM is too low at WOT, try a reduced pitch and retest.

7. If the RPM is too high at WOT, test a propeller with more pitch. One pitch size usually

results in a change of 200-300 RPM.

8. If the boat will be used for two applications, like water skiing and cruising, it may be necessary

to test propellers for each type of use.

[FONT=Arial,Bold][FONT=Arial,Bold]Step Three: [/FONT][/FONT]Adjust the engine mounting height for peak performance.

The ideal engine mounting height provides the fastest boat speed without sacrificing acceleration,

maneuverability, or engine water pressure. It is achieved by a trial and error method.

1. Start with the outboard’s anti-ventilation plate even with the bottom of the boat.

2. Raise the outboard one mounting hole at a time until engine performance is no longer

acceptable. Then, lower the outboard back down one hole.

Optimum engine height is affected by propeller style, diameter, and pitch. As you raise the engine, you

may need to experiment with a variety of propellers to maximize performance.

 

[danintruder]

bullet , i agree with omc and doing it that way pretty much , the boat is very afected by fuel weight in front like i read on an older post here .
the reasons i want to try the 23 is : i'd like to reduce wot rpm to 5500 and when doing 54 mph at 5800 the trim is topped out ! i think with 23 it should do it with maybe some room to trim more .

 

[Teamster]

i have a 21 pitch on it now at 5800 i want to lower rpm ence the 23 .
pressure guage says 31 at wot now so pretty good .
only one hole left .

Going to a 4 blade would drop the rpms some,..

If you know someone that has one you cold borrow it would be cheap and easy to find out.

 

[Teamster]

Whoops sorry,...

I missed the part about the trophy 4 blade already on it,..

Then I'd go to a 23 pitch to drop the rpms,...

 

[Faztbullet]

What hurt those Stratos series is fuel tank behind passenger/driver as effects running attitude(weight forward) and pad is not flat. A good 24 Raker/Turbo should put you in low 60's.....(ps not all Rakers are good)

 

[danintruder]

fuel tank is 5 ' in front of driver

 

[Sea Rider]

I'm sorry, but you are sadly mistaken by a long shot. Water flow should be just under the av plate. That's the bottom plate, not the top plate. Look at your first picture, and see the spray coming way out to the side. That engine is mounted way to low and they are losing at least 10% in speed and fuel economy. And the second photo is just wrong.

Touché, you're sadly mistaken too by far, if you have a close look at the posted pics on post 7 you'll see that aft of middle plates are located 2 large horizontal cuts to both sides of lower leg which are part of the exhaust system and must be fully covered with water when combo is on its way or will dump excessive exhaust noise to the open air when engine is running at full range. Anything to say about what you have olympically bypassed.....

With respect to first posted pic which BTW is not mine and for my particular lower leg height taste would sit OB half an inch lower along a better trim angle for plate to run parallel to water level to flatten and smooth out the small side splashes seen there. Seeing a posted pic says nothing as you can't possibly know at which rpm is the engine running at or if has gone through a prop maximization to state theoretical 10 % loss.

Happy Boating

 

[WesNewell]

Read this on boats dot com
https://www.boats.com/how-to/the-ou...peed-with-outboard-engine-height-adjustments/

 

[WesNewell]

[No message]

 

[Sea Rider]

To end this post on my side and for tech clrafification purposes : Should know there are 2 type of outboards :

Jurassic ones in which the upper to lower plate distance is just 2 inches in height with a much larger gear case lenght measured from AV plate to lower skeg. Prop is way downward from AV plate.

Modern ones in which given distance is 4 inches in height and the gear case is much shorter in lenght measured from AV plate to lower skeg. Prop is nearer AV plate compared to Jurassic ones distances.

The OP's one is the Jurassic model, although can play with posted pic literature regarding flow passing at different lower leg heights, on HIS particular one must pass right under upper plate in order for sided cut slots which are not mini torpedo tubes to work fully submerged and part of the exhaust gases be exhausted against water along doing their homework right as were factory designed to work.

On Modern 2-4 strokes ones can't possible play with same lower leg heights as with Jurassic ones as will have excessive prop aeration if flow pases around AV plate area due to having a very low water bed heigh for prop to grip spot on achieving top prop thrust. Are we on same page ?.

Happy Boating

 

[WesNewell]

The jurassic period started 201 million years ago and ended 145 million years ago. If you find an out made during that period, please advise of make and model. The above photo pretty much shows all that is needed for outboards made in the last 60 years.

 

[Faztbullet]

Sea is just uninformed on hull design and HP set-ups. Rubber boats totally different than FG hulls. Johnson/Evinrudes V-6 lower have always been 8" from center of propshaft to AV plate..from 1976 to 2015. The V-4's always been 7-1/4 except for batwing unit and same(8") for the Merc V-6 units 1976-2018. Don't see a Jurassic year in there.

 

[Faztbullet]

This is how it should run or close to it...…
https://www.youtube.com/watch?v=bTFuihWZ__A

 

[Sea Rider]

WN, you always like flying over the cukoo's nest when don't have an appropriate tech answer to give or even evade about what posted.. Congrats, now with very few post you've turned into the new Guru of the of the OB-Transom Height Optimization and Prop Maximization for all type of OB's.

Fazt, stop pigeonholed me in that only worked with inflatable boats, been doing OB-transom modifications, optimizations along prop maximizations on many type of boats for the last 10 years, not with Jurassic ones thought which you guys love boating with.

What find really amazing is that you guys always give for granted what's been posted for years now about lower leg heights, which are complete different installations between Old and New OB's which you like throwing in same sack.

As we will never will be in plain accordance about this issue, keep rigging your OB's with flow passing by AV plate and I will keep mines rigged with flow passing right under Upper Plate....

Happy Boating

 

[QBhoy]

Just catching up on this....
searider are we thinking that the water flow should be in line with the upper fin ? Surely not ? Should be right on the cavitation lower plate at best position, with the lower cav plate being in line with the most lower line of the hull.
Is there a difference with older engines ? Is that what we mean ? Sorry...my iPad is very slow and takes an age to read through all the posts.

 

[WesNewell]

I hate typing because I'm not a typist, but here goes. It's a simple matter of hydrodynamics. An outboard mounted too low induces drag from the lower unit. This causes the need for more power to achieve speed. And since the engine is at max, the rpm is also reduced as a result of the extra resistance. Now if you try and trim the engine up to reduce the drag, it causes the trust of the engine to also push downward on the stern of the boat thus raising the bow. Once the bow has lifted enough to get the trust of the prop in parallel with the water, the bow will then drop back down and this cycle continues giving you a porpoising effect. And now if the engine is mounted too low, the shape of the av plateb eing wider at back exacerbates the problem by forcing the stern of the boat down faster as it is below the waterline. And if you have a jackplate, this just makes it worse with the engine mounted to low since it acts like a short lever putting more downward force on the stern. There's only one way to do away with this problem and that's to mount the gine so the AV plate rides above the waterline at planing speeds. Even then you may experience some porpoising if you trim the ngine too high, but it will bee much less severe than with the engine mounted too low. There are a lot of hull designs from almost flat across to deep v's, On flat and modified V's the AV plate in line with the bottom of the boat should give you a good smooth with little loss in speed or fuel economy. For a deep v hull you will probably want it an inch or more higher. And even higher if you have a jackplate. And if you have an aluminum boat with a stringer about an inch wide and deep down the center like mine, you'll wnat the av plate evn with the top of that V or maybe a little higher.

 

[QBhoy]

My boat runs along both upper and lower lines. Cruising it looks as if it on the upper and trimmed out at WOT with the boat running up on its pad (on bottom and rear of hull) it’s on the bottom AV plate with prop surfacing a little. As we know some props are designed to have a little surface on boats like this. See link below. Feel sorry for the wee Alpha one running likely a wee bit over 300hp through it !

https://youtu.be/MF-v2LYlogw

 

[Faztbullet]

which are complete different installations between Old and New OB's which you like throwing in same sack.​

What is meant by this(above) as 1976 to 2018 lower units haven't changed AV height. Now Tohatsu may have but Johnson/Evinrude/Yamaha/Force/BRP have not....Explain with numbers. I only offer insite to some of your posts as the information you posted is not totally accurate.

 

[Sea Rider]

Hey guys, lets stop high lacking the OP thread. Give me some days and will open a new post in which will deeply discuss what has been a long time quarreling about best lower leg height in tech terms.

Happy Boating

 

[Sea Rider]

Faztbullet, WesNewell, Qbhoy, Scott and any other member are invited to join the "Lower Leg Plates" conversation...

Happy Bpating