Motor Height?

[Bozo]

I was getting more porpoising than I wanted so I raised my motor up a hole.<br /><br />Before I raised the motor I was able to acheive a RPM of 5800. Now that it has been raised, it only goes to 5400 WOT. <br /><br />I would have expected the opposite to happen and to have been able to acheive a higher RPM with less motor in the water. <br /><br />Is this because I was getting more slip with the added resitance of more motor in the water and now I am getting hooked up better? <br /><br />19 pitch prop on a 115 Yamaha. I think the recommended RPM for this motor is 4500-5500. Would anybody recommend going with a higher pitched prop to get it more in the middle of the recommended range?

 

[ob]

Re: Motor Height?

Was there some duration of down time betweeen motor being able to achieve higher rpms before you raised it?The reason I ask is that as you figured ,it should rap out higher,and I was thinking that maybe your carbs have fouled a bit since last tested.

 

[Bozo]

Re: Motor Height?

Nope. All done on the same day. The only thing I can figure is the prop slip.

 

[ob]

Re: Motor Height?

That one is by me Bozo.Maybe Dhadley or some other setup guru will come around shortly.

 

[Dhadley]

Re: Motor Height?

Got me too ob. More slip equals more rpm but usually less speed. More height equals less drag equals more rpm. <br /><br />The only thing I could think of was that somehow something got moved, knocked loose or otherwise changed and now the carbs are not opening all the way or the timing doesnt advance all the way. <br /><br />I guess the real test would be to lower it and start over. Not a solid answer but we'll all think on it and I'm sure it'll get resolved.<br /><br />Keep us posted!

 

[Dhadley]

Re: Motor Height?

To answer the other question - 5800 is a great rpm. It should have a decent holeshot and good top end.<br /><br />This is the EXACT same prop - right? Not one like it or repaired or anything else - right? The only thing changed was the motor height - right? Same spark plugs, same fuel, same load, same fuel tank and same ammount of fuel?

 

[Bozo]

Re: Motor Height?

Yes. All is the same except the height.

 

[ob]

Re: Motor Height?

And were both water tests done in the same water.Salt/fresh?If so ,I would check as Dhadley suggests the carb flaps with the intake cover removed and insure they are fully opening.I'm still stumped.Wouldn't happen to be a wad of fishing line around the hub?

 

[Bozo]

Re: Motor Height?

Dhadley, you hit on something that gave me a clue to check. It is running faster at the lower RPM.<br /><br />I hadn't thought about checking the speed. I don't have a speedo on the boat and took it out this evening with my handheld GPS and it is going about 4 mph faster at 5400 rpm's than it used to go at 5800 rpms. It has to be slip.<br /><br />Thanks.

 

[lakensea]

Re: Motor Height?

I would say you have a tachometer problem. If you had prop slip to begin with, and raised the engine, that would make it slip more. Does the engine sound like it's lugging at 5400? Does the boat snap up on plane any better in the higher engine position?

 

[bonitoman]

Re: Motor Height?

Are you trimming your motor up all the way when running to get the maximum rpm before it overrevs?

 

[Dhadley]

Re: Motor Height?

When you moved it up did you move it back also? IE - did you raise it by adding a jackplate?<br /><br />Less rpm = less slip. Since the speed increased and the rpm decreased the slip has decreased. There is something stopping the motor from wot but yet the prop is getting a better bite.<br /><br />I like a good mystery!

 

[Bozo]

Re: Motor Height?

No, I didn't add a jack plate or anything. I made the change at the boat ramp. <br /><br />I pulled the boat out of the water on the trailer, unhooked it from the truck and tilted the motor down. I then used the tongue jack to adjust the height of the motor after removing the top bolts from the motor mount and loosening the bottom bolts. I would crank a little and then go back and check the alignment...went back and forth making slight adjustments until the motor lined up properly, reinserted the bolts and tightened. Pretty much a painless process. <br /><br />The boat rides better now even though the rpm's dropped. Therefore, I really don't think it is anything mechanically different with the carbs or prop. The prop never touched the ground, only the skeg when adjusting. I inspected the skeg and there appears to be no damage from the adjustment.

 

[Dhadley]

Re: Motor Height?

What a flash back! Been there, done that! Lol!<br /><br />I'm at a loss to say why you lost rpm by raising the motor. But I'm glad you gained speed. Can you rais it any higher?<br /><br />Watch the water pressure!

 

[Bozo]

Re: Motor Height?

Well, actually I started off by going up two notches and trying it but, if I trimmed it out any at all it would start to cavitate and the speed was way down. Even without a speedo I could tell that I was only going about 30 mph max compared to the 40 mph prior to the adjustment.<br /><br />I moved it back down a notch and then that is when it seems to be in the sweet spot but, the rpm's are down 400. Using the GPS yesterday I am now able to achieve a speed of 44. <br /><br />So, at 5800 rpm's it will go 39-40 with the motor in the origanal position, moved up a notch it will turn 5400 but will go 44 mph.

 

[Dhadley]

Re: Motor Height?

Ok good. Going all the way up was too far. Thats good info and a great way to test. <br /><br />Its obvious the prop has hit a sweet spot but I would like to see your r's get back up there.<br /><br />When the motor was down, where was the cavitation plate in relation to the bottom of the hull? Was it below?

 

[Bozo]

Re: Motor Height?

It was about 1/2" below the hull. Now it is right at or just above the low point of the hull. <br /><br />The boat is a custom 19' fiberglass hull made by Holmes Boat Works in Houston. The were mainly known for the wooden boats but, did produce a few fiberglass models in the late eighties/ early ninties. I would guess less than 20. <br /><br />The hull is designed for bay fishing and is a v hull but, it is relatively flat on the bottom in comparision to most models of its class.<br /><br />With that in mind, it may not "act" as expected under simular conditions on other models. <br /><br />I just with I could get the extra R's back.<br /><br />I also have a stingray hydrofoil on the cavatation plate which was my first attempt at solving the porpoising problem. <br /><br />Do you think I should pull that off and see what difference it would make?

 

[Dhadley]

Re: Motor Height?

OK. Heres a possibility - since the motor was so extremly low it could have been "aerating" the water. We see this a lot if someone mounts a speedo pick up or a transducer too close to the motor. The prop runs in water with air bubbles and has more slip. Its posible you may have moved the cause of the turbulance and thereby reduced the slip factor. Not a great answer but perhaps....<br /><br />The fin being that low will definately cause problems. Not the least of which is drag. I would certianly try it without the fin but most likely you may like it better on.<br /><br />Water should have been shooting up each side of the motor too. If it was thats probably gone now. <br /><br />I dont remember what type prop you have but I think you are on the right track to start playing with that. You could extend the cup and try it at the higher X dimension. If it an aluminum prop you could try a ss prop and the higher X dimension.<br /><br />Never ending, huh? I ended up with a 20' Hydrostream that I sold and rigged new in 1985. I'm still trying stuff.<br /><br />Good luck!