Where did my rpms go?

[SnookHunter]

I have a 05 16' Lowe Sea Nymph with a 05 Yamaha 25hp 4 stroke. I put a jack plate on over the summer and now av plate sit about 1/2 inch above the bottom of the boat. I had a Michigan wheel 9 7/8 x 10 1/2 and the top speed was 26.4 with gps and the rpms were around 6210 on a light load. With the boat loaded with 3 guys and fishing gear the boat still went 23 mph at 6110 rpms. I wasnt comfortable running wot with a light load at 6210 rpms. So I changed props to a Solas Amita 9 9/10 x 12. Now my rpms took a big hit. With a light load the rpms are about 5600. And 5490 with full load. My speed did go up a few mph as well. I lonly went up 1 1/2 inches in pitch. The boat has no problem getting on plane. I also have a stingray hydrofoil on it to. It does appear the Solas is cupped, while my old prop wasnt. Would this cause the big hit on rpms? If I stay with the Solas are rpms in the 5400-5600 range ok? Is this lugging the motor? Tiny tach was used to check rpms.

 

[walleyehed]

Re: Where did my rpms go?

How much set-back did the jackplate add?

 

[SnookHunter]

Re: Where did my rpms go?

It added 5 inches of setback.

 

[walleyehed]

Re: Where did my rpms go?

OK, for 5" set back we go up 1" just to get back to "zero".
If not already, I'd try the second set of holes from the top, and get a measurement there....I think the third set of holes is where you will end-up, it may even allow 4 ....depends on water pick-up and prop bite.

 

[SnookHunter]

Re: Where did my rpms go?

The jack plate is up as far as it will go. Are you referring to the pin that allows the motor to be trimmed? If so, I'm already at the 3rd set of holes. Maybe I should try the 4th?

 

[Texasmark]

Re: Where did my rpms go?

Not trying to butt in, but here's a tidbit of info...BTDT.

Not familiar with either prop. I had a SS prop that was causing "clutch dog rattle" (yeah I know you guys have heard this 1000 times.....heres 1001 )

I thought I'd get an alum prop to help to or eliminate the rattle.

In short, the alum prop had 1" less pitch and instead of increasing my rpm's (by a 150-200) it cost me 600 rpm's. The only thing I can see that was the real culprit was the blade thickness as both had all the goodies found in a high perf prop and the SS had very thin blades.

So, when you get through getting your setup right, you might take another look at your props; or the difference between them.

HTH

Mark

 

[walleyehed]

Re: Where did my rpms go?

Where your engine mounts to the jackplate there are numerous holes, vertically. If the jackplate is "fixed"-just a set-back plate, we need to raise the engine by the mounting bolts- or the jackplate if it's a movable plate.
We need to get the engine up, not OUT, as with the tilt pin in the third hole.

 

[SnookHunter]

Re: Where did my rpms go?

Well, I took the boat out today with different results. I moved my tilt pin from the 3rd to the 4th hole and loaded the boat with all my fishing gear an filled the livewell with 7 gallons. To my suprise at wot the rpms are 5790 now. This was with a slight chop. So if its calm it might go to 5800 over a little over. Top speed is now 28.7mph. Are these results acceptable without lugging the motor? Do I need to swtich props?

Also, my jack plate is a bobs machine mini. It doesnt have any holes to move it up or down. You just loosen the bolts and slide it up or down and then retighten. Its as far up as she will go. And its mounted up as high on the transom as it can be. My stingray hydrofoil is running out of the water at wot.

 

[steelespike]

Re: Where did my rpms go?

I couldn't find The specs on the 25 but the 20 hp max rpm range is 5,000 to 6,000 rpm.My guess is the 25 is slightly lower. I would think your motor is happy in the range you are running it.Though I would keep it around
5,700 when empty.I would think your speed is about the max with the load you are carrying. I would guess the motor is rated for about 30 or 32 stock under ideal conditions and load.You may find your boat runs slightly faster in a light chop than glass smooth water.

 

[SnookHunter]

Re: Where did my rpms go?

The 25hp max rpm range is 5000-6000 as well. I'm totally satisfied with the speed, I just want to make sure the rpms arent lugging the motor.

 

[Crownie2]

Re: Where did my rpms go?

IMO you are not lugging - sounds just about perfect...

 

[walleyehed]

Re: Where did my rpms go?

I'm still in ahhh that it will plane out and not porpoise in the 4th pin-hole.............

 

[SnookHunter]

Re: Where did my rpms go?

Well you are correct. It will plane out just fine but anything over about 18mph without somebody up front it purpoises terrible. It always has since its so stern heavy. But since I mostly use the boat with 2 people its not a big deal.

Well after fishing today, I realise this prop just isnt gonna work. As soon as I added a second person the rpms are back down to around 5480 again. The motor just doesnt sound as good as when she could turn 6000+ rpms.


Walleyehead, I'm looking at the Stiletto Triad 10 1/8 x 11. Will this prop get my rpms close to 6000 again and offer some good performance? Or should I look at the Triad 10 1/8 x 10. I have read that a good stainless will pull the rpms down some even if you use the same size prop. Is this correct? The propeller that came with the motor was a 9 7/8 x 10 1/2. And this size alum. prop lets the motor turn a hair over 6200 rpms. So would a stilleto be a wise choice? Thanks for you time.

 

[walleyehed]

Re: Where did my rpms go?

Stiletto is always a wise choice in my book....
Question I have is; "Is the engine MOUNTED as far up as the mounting holes on the engine, will allow"...not the holes on the plate, but the holes on the engine mounting bracket where the bolts go through...is this as high as it will go? The tilt-pin needs to go back atleast 1 hole, maybe 2....
The total available height of the engine will determine which prop we can use.

 

[SnookHunter]

Re: Where did my rpms go?

If I'm reading you correctly no the motor is not mounted as high as it will go. The engine mounting bracket is resting on the jack plate and then I drilled the holes through the plate. I would say that the top 2 engine mounting holes are an inch from the top of the jack plate. I also must say with the motor at its current height loses its bite in turns.

Are you thinking I should redrill my engine mounting holes?

 

[walleyehed]

Re: Where did my rpms go?

If you drilled the holes with the engine bracket arms resting on top of the plate, you can remove the bolts, lift the engine to the next set in the engine bracket-not the plate-and this will require a higher tech prop, such as the stiletto, to stay hooked-up. If you want, email me and I'll give you the links of some sites that offer test-props so you don't have to buy several to get it right.
ktsander@st-tel.net

 

[Dhadley]

Re: Where did my rpms go?

Remember this is a 4 stroke. The r's at wot have to be at 6000 or so. If not, you'll lose a lot with a small increase in load - as you've seen. Same with increased pitch. Stick with Kenny as he walks you thru raising the motor. There's a very definate method to his madness.

 

[SnookHunter]

Re: Where did my rpms go?

Ok, I just want to make sure I undestand this before I do it. I have 4 holes in the engine mounting bracket. 2 top and 2 bottom. So I need to remove all 4 bolts and lift the motor so that the bottom 2 hole line up with the top 2, right? Ok, if this is true the bolts are only 5/16. Will these be enough to hold the motor safely?

 

[Crownie2]

Re: Where did my rpms go?

Oh - oh.

He is talking about raising the engine 3/4".

 

[walleyehed]

Re: Where did my rpms go?

yea, and that could net anywhere from 75-150RPM....
Go up a hole if you can.
You may have some blow-out, but I'll send ya the links if you email me on where to send you for test-props.
addy is in my above post.