RPM/Engine Height/Prop Questions

[100333624]

Hello this is kind of a mixed topic I guess, I am looking for your advice for those who are familiar. I have been in the process of propping my rig properly. I am currently running a Laser 2 in 20" pitch and was recently getting only 5000-5100 rpm with all my tests on that prop. After careful observation of the anti vent plate I decided to raise the motor, so I recently raised it one hole up. When leveled out the boat bottom to the anti vent plate, it currently is sitting 3/4" above the bottom of the transom and I still have one more hole that I can raise it. After doing 2 test runs with the new height, the first got me about 5300 rpm, BUT the second test today got me to 5500-5550 rpm trimmed out?? The thing is I had 2 completely different trim results today from last week. Today it seemed like I could trim so much out and I did... It just kept going and going and I hit 79 kph, the other day it was a touch slower by 1 or 2 kph, but I didn't notice the trim out this much? I usually trim to where the boat starts bouncing, then I trim in just a touch, but today I could trim out so much I was scared maybe I'd pull the prop out a bit and over rev the motor? I did notice the steering was very stiff while trimmed out too, and I have hydraulic steering. The conditions today were a little less windy than last weeks test, and so less choppy, could that be the reason?? It just seems that once you think you start to learn your boat, there comes a curveball somewhere to throw you off...

I thought I was going to have to get rid of my prop because I wasn't getting enough rpm but after todays results I guess its the right prop for me? I noticed since switching motors (used to have a 50hp 4stroke, now have a 115hp 2stroke) my gas mileage has almost doubled or tripled?!? I was used to filling up about $30 every trip with the old motor, and up until 2 weeks ago I over filled my tank (100 liters) and so I had a little extra weight to my first test runs. After about 3 or 4 fishing trips out I still have over half a tank left?!?!? I will admit I don't drive around a lot but when I do I'm going full speed for quite a few kilometers. To add to this, after todays test run while looking back at the motor as I was trimming out slowly (I trim out a little bit at a time) I noticed some different splashing going on which caught my attention, I can only think it was the anti vent plate or most likely the other plate above it throwing off water at a high angle behind the motor in a V shape (NOT a rooster tail) one line to the left one to the right. When I did another run to check it out, I told my partner to hold the wheel while I went back to see and it wasn't doing it now, so it could have possibly been weeds maybe? Very strange as its never happened to me before. But I noticed my anti vent plate was still buried in the water so I think I have to come up one more hole which will be the maximum height I can raise it, and the plate will be 1.5" above the bottom of the boat. Now I gained like 400 rpm from raising it one hole, whats raising it another hole going to do?? Will I have to go up in pitch to a 21? This stuff is so weird and strange, I was certain I had to go down in pitch to increase rpms before thinking about raising the motor, but seems like it's going the other way around?? I am afraid if I go up in pitch the holeshot isn't gonna be that great, its already border line OK, or do I just keep this 20 pitch and add some more cup to it to bring the rpm down a bit and better bite?

PLEASE give me your advice, I am eager to hear from you who have been in this situation.

Thanks!
Nick

 

[SkiDad]

You seem to have the correct prop and height. I would leave it.

 

[100333624]

You seem to have the correct prop and height. I would leave it.

My anti vent plate is still buried under the water at speed though? Doesn't it cause more drag when buried, should I leave it like this?

 

[steelespike]

Your anti vent plate height would vary from boat to boat even prop to prop.Typical load.I was surprised to see one the other day clearly out of the water.
The best all around performance would determine the best results.

 

[Sea Rider]

Can't dial a nice engine height to any particular combo by guessing or assuming it's right. Post a pic shot sideways of your lower leg to have a look at both both plates configuration. Was the test done with you alone and bare bone combo and least fuel or just slightly loaded ? What do you want to acccomplish, solo boating, family oriented, skiing, pulling water toys ?

What's the min-max rpm wot range for that OB ?

Happy Boating

 

[100333624]

Can't dial a nice engine height to any particular combo by guessing or assuming it's right. Post a pic shot sideways of your lower leg to have a look at both both plates configuration. Was the test done with you alone and bare bone combo and least fuel or just slightly loaded ? What do you want to acccomplish, solo boating, family oriented, skiing, pulling water toys ?

What's the min-max rpm wot range for that OB ?

Happy Boating

Here are some pics of it on the trailer, I can take some water test photos maybe on Friday, I live 10 mins from the lake so its not a problem. All tests were done fairly consistent with me and one other person on board, light gear, and it started with a full tank of gas, now there is a little more than half tank left. My goal is to accomplish the best overall performance, I am somewhat happy with the top speed I guess, no towing, no water sports etc, I use it for fishing only. Holeshot is ok, I can live with it, it would be nice to get a little more top speed out of it if possible. I keep my boat fairly light and don't have things that are not needed on board. I am most likely going to be running with a half tank of gas or less as it seems to last me forever, why have the extra weight on board right? Engine is a 1995 Yamaha 115hp 2stroke, max rpm is 4500-5500 so I am already there. Question is the steering felt extremely stiff for some reason and I have hydraulic steering? And do I need to raise motor one more hole up as I cant see the anti vent plate when under way? If I do raise one more hole up, I will need a new prop I am guessing as I gained almost 400 rpm just raising it up one already?

 

[100333624]

Oh and just to mention, the stiff steering doesn't pull left or right, its dead straight but just makes it hard to turn even with 2 hands... could be trimmed too much up or..??

Looking forward to your replies. Thanks!

 

[Frank Acampora]

Look elsewhere. generally, trimming up lightens the steering. Engine height will vary with the hull and design. The only way you know you have it correct is by experimenting. Raise the engine another hole. If it blows out on turns or even the straight when you feed up trim, then lower it back down.

The absolute best you can hope for is that the anti-ventilation plate is riding at the water surface. But remember: You are not racing. An inch too much will not make that great a difference.

 

[Sea Rider]

Ok 1033, before optimizing your current engine height and what need to know, will need to modify one of your pics for clarification and plain understanding. Meanwhile will like to share this set up with the audience as an example about the importance of achieving best engine/transom height to avoid unwanted performance issues. At a simple pic glance down below, how do you find this engine set-up, would you say it's correct, combo will have excellent water performance? see any flaws, inconsistencies somewhere, comments are welcome.

Scenario :
2 stroke Merc 150 HP powering a Four Winns 4.85 mtr boat for OB use. Don't ask me about wot rpm, hole shot, top speed, overall performance, this combo is under dry evaluation right where combo is parked. Oh boy, love challenges and end results comparing plain theory versus plain practice once on water.



Happy Boating

 

[Frank Acampora]

No Title

This is a racing application. notice where the plate is in relation to the tunnel.

 

[100333624]

Look elsewhere. generally, trimming up lightens the steering. Engine height will vary with the hull and design. The only way you know you have it correct is by experimenting. Raise the engine another hole. If it blows out on turns or even the straight when you feed up trim, then lower it back down.

The absolute best you can hope for is that the anti-ventilation plate is riding at the water surface. But remember: You are not racing. An inch too much will not make that great a difference.

Please take a look at this picture. This is the only plate that is visible while moving. I am afraid that even raising it another hole to the maximum height still won't make the anti vent plate visible since its only a 3/4" difference? I wouldn't mind trying it though. Generally speaking my steering is a little tight, but easy enough to steer any direction, but this time it was almost impossible to steer? I noticed before I had a tiny leak in the helm underneath, just a few drops, but other than that nothing else, and hasn't leaked since the winter. I installed the unit myself per instructions, everything is new EXCEPT the sea star tilt helm. I hear they usually leak and need rebuilding so I may consider getting the gasket kit and doing it over the winter. I would just like to nail down the engine height and prop and rpm before storing it for the winter so I can work on other things for next season.

Also, I have maybe a half inch between the prop and the trim anode behind it. Do you think that the prop being so close to it (since it is a high rake prop) producing enough thrust and forcing it on the anode making it difficult to steer? I noticed I still had stiff steering even with the aluminum props I've tested at speed, but never as stiff like yesterdays test run. Or could it be that because I raised the motor, the splash coming off the bottom of the hull at the transom is at an angle making it more difficult to steer the boat? The steering is effortless when going slow otherwise, only at high speeds its very stiff, and only with this Laser 2 have I noticed it so stiff, so I don't suspect the helm or the cylinder.

 

[Frank Acampora]

I have no experience with hydraulic steering so I can not help you with that. However, try lubricating the yoke and kingpin. It could be as simple as engine thrust against it is causing it to bind.

As far as engine height, do what the hull is telling you: Raise the engine one more hole and see what happens. I can not see the top of the transom but if there is room, you may need to drill higher mounting holes if the engine wants to be higher than that.

The alternative is to buy or fabricate a jack plate for ease of raising and lowering the engine.

 

[Fed]

I'd lower the motor a hole to raise the bow which will reduce the keel length in the water and make it easier to turn.

That's the internet for you, plenty of opinions.

One thing I do suggest is to only do one thing at a time between tests, I'm looking forward to see how this one pans out.

 

[100333624]

I have no experience with hydraulic steering so I can not help you with that. However, try lubricating the yoke and kingpin. It could be as simple as engine thrust against it is causing it to bind.

As far as engine height, do what the hull is telling you: Raise the engine one more hole and see what happens. I can not see the top of the transom but if there is room, you may need to drill higher mounting holes if the engine wants to be higher than that.

The alternative is to buy or fabricate a jack plate for ease of raising and lowering the engine.

Its interesting that you mention that because as I was looking over the motor the other day I looked over at the yoke and saw what looked like a little more than usual grease coming out of there... Its solid nonetheless in all directions.

I am not sure what the boat is telling me to do, I just want to do what will make it perform the best it can. I will admit, it had better bite when I had it one hole lower, and I only gained 1 or 2 mph raising it, but I can not believe rpms went up almost 400? Holeshot is pretty much the same too. I am willing to try it out on the highest hole just to see how it performs, that way if I have to move it back down at least I tried it and it wasn't right.

Also, is there a general rule to outboard height for center consoles? My boat is only a 16.5' center console (Campion 165) its pretty deep for a 16 footer and it obviously has a deep V hull. Is there a general rule to how high motors like to run on these hulls, as opposed to other hulls like bass boats and such? Like what do most center consoles running a light to medium load have their engine height at?

 

[steelespike]

W0191221 on't know if its just me but I had to search and search for the key information prop size brand and year of motor,hp, size and type of boat.Performance numbers.
I converted speed to mph. 49.09 .Any boat in the 50 mph area is doing really well.
Calculator indicates 5% slip using 20" pitch Calculator suggests to test ss prop use 21" pitch. 21" indicates 10% slip a more normal slip number.
​ Fuel consumption at wot the 50 would be at least 6 gallons per hour (gph) @ wot. The 115 at least 14 gph @ wot.
Obviously the 115 is a lot faster than the 50. GPH is the same whether pushing a houseboat or a runabout.
MPG is the result of the speed achieved.Thjese numbers assume the motors are carbed.
Again motor height can vary same model boat to same model boat, same engine,prop etc.
Load placement and amount.

 

[Sea Rider]

10033...

Were the 3 posted pics from your 150 HP in actual use ? If so, which OB brand and leg size is it ?

Happy Boating

 

[100333624]

W0191221 on't know if its just me but I had to search and search for the key information prop size brand and year of motor,hp, size and type of boat.Performance numbers.
I converted speed to mph. 49.09 .Any boat in the 50 mph area is doing really well.
Calculator indicates 5% slip using 20" pitch Calculator suggests to test ss prop use 21" pitch. 21" indicates 10% slip a more normal slip number.
​ Fuel consumption at wot the 50 would be at least 6 gallons per hour (gph) @ wot. The 115 at least 14 gph @ wot.
Obviously the 115 is a lot faster than the 50. GPH is the same whether pushing a houseboat or a runabout.
MPG is the result of the speed achieved.Thjese numbers assume the motors are carbed.
Again motor height can vary same model boat to same model boat, same engine,prop etc.
Load placement and amount.

Are those numbers you mention for my setup or someone else's? 5% slip or 10% slip which is better? I was thinking of raising one hole and trying a 21" pitch prop to see what difference I get.

 

[100333624]

10033...

Were the 3 posted pics from your 150 HP in actual use ? If so, which OB brand and leg size is it ?

Happy Boating

Sorry I'm not sure I understand what you mean by we're they in actual use..?? And the motor is a 1995 Yamaha 115, not 150, it's a 2stroke longshaft.

 

[steelespike]

Are those numbers you mention for my setup or someone else's? 5% slip or 10% slip which is better? I was thinking of raising one hole and trying a 21" pitch prop to see what difference I get.

5% is unlikely If right would be amazing. 10% is more typical and likely to be more accurate.
Slip on the typical planing hull would be about 12%. could be a little less but 5% is unlikely. It was based on the actual dimension of your prop 20"
If we calculate using 1" additional to calculate for a SS prop. we get a more usual slip.The results of the calculations is only as accurate as the input.
The tach is always suspect when the calculations doesn't seem right.
Wot GPH of any carbed marine engine can be estimated by multiplying by 12 to 14% thus a 100 hp would use 12 to 14 gph at wot.
The wot gph is roughly the same whether on a houseboat or a runabout.

 

[Sea Rider]

OK, it's a 2 stroke 115 HP Yam.

Need some information about last pic posted data :
- At which trim position were you when water flow passed right under upper plate, trimmed down, middle, trimmed up ?
- Was load well distributed on deck to achieve a well balance combo.
- How much under upper plate was water flow at speed passing bye ?
- How did you check the posted height while water flow was passing through lower leg.
- Popped you head out transom, installed a go pro cam on transom facing downwards, someone was having g a look from other boat ?

Happy Boating