First boat... Need help with low rpm/speed

[Smgbad]

Hi all. Just bought my first boat... 2016 16' Grumman Cayuga being pushed by a 1994 Nissan NS40c. 40hp 2 stroke.

A little background. The boat is typically run within 100-300lbs or so of maximum capacity (1200 lbs)a as i am a big boy (450 lbs) and I typically have 1 or 2 passengers and gear. I sit at the console (located middle of the boat to the starboard side)

The issue I'm having is that the boat (at WOT) will not plane. It'll get close when I have my passenger sit in the front (closest I got was a top speed today of 13mph per GPS). When a passenger sits in the back I get a top speed of about 8mph.

The tach (induction with the wrap around wire) shows 960(ish) at idle, which corresponds with what the factory manual says it should be. WOT however only produced 3150 and 3530 on two different runs going opposite directions. Motor is manual trim, so I let it sit down at the second mounting hole (counting from the one nearest the transom).

I currently have a 12" pitch prop on, but am thinking it may be either a spun hub or just a dramatically oversized prop for the heavy load that the boat is lugging around (ie: my butt).

Any ideas or advice would be greatly appreciated before I spend money that need not be spent. If it IS the prop, it looks like they make a 9" pitch which is called for under "heavy loads". From what I'm reading, that would still only get me to around 600 more rpm, when I'm nearly 2,000 short though. The engine starts and runs fine, and is running nonethanol at the moment.

 

[QBhoy]

Not too familiar with these engines, but before looking at props for now, does it have power trim ? If so is the trim all the way down when trying to plane ?
If no power trim, you might try to adjust the trim manually at the engine to lower it a little.

 

[Smgbad]

It's manual trim. Can't steer the boat and move back there to raise or lower it as I am a big dude and the boat shift quite a bit when I walk around on it. It sits at the (almost) lowest possible position when I have the motor going. I keep it at the second hole from the transom on the mounting holes. I could lower it the full amount but have never tried that.

 

[Faztbullet]

Either motor is not opening up or toooo much capacity. Check cables to see if timing and carbs are operating correctly and reaching WOT setting. Also let a lighter person operate the boat, if it does OK and reaches RPM spec you will need to drop prop sizes due to load....

 

[Smgbad]

Will do... how does one check the cables for timing and carb operation, if you wouldn't mind elaborating for a novice?

I know the cables going to the remote throttle assembly appear to be working correctly if that's what you mean.

One odd thing I ran into (keep in mind that this is my first outboard motor) is that when using the muffs, the prop will turn freely in neutral. Meaning that it turns and you can touch with one finger and it will stop. When shifted into forward it reverse it goes into gear fine and starts spinning due to being connected to the proper gear. Is it normal for it to free spun in neutral though? Just figured that may be a clue...

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[WesNewell]

Yes, the prop should spin when not in b gear. First. Your engine should be mounted where the av plate is in parallel with the hull bottom of the boat or with manual adjust like you have, at the least angle that will produce bow lift. When down on the stop, the av plate should not be below the bottom of the hull. Assuming the boats dry weight is ~420#, with a 1200# load you should still get over 25 mph with a 12P prop and a wot rpm range clo0se to the top range. This assumes your engine is in good working condition.

 

[Smgbad]

Yes, the prop should spin when not in b gear. First. Your engine should be mounted where the av plate is in parallel with the hull bottom of the boat or with manual adjust like you have, at the least angle that will produce bow lift. When down on the stop, the av plate should not be below the bottom of the hull. Assuming the boats dry weight is ~420#, with a 1200# load you should still get over 25 mph with a 12P prop and a wot rpm range clo0se to the top range. This assumes your engine is in good working condition.

Yeah... I'm getting 13mph with someone at the bow for weight. 8mph if they are sitting aft. I'll take pics of the AV and where the motor is sitting while im the water to see if that may be part of the problem.

Would it be a bad idea to go ahead and get an 8" pitch 4 blade aluminum prop and hub coming? If not that I was thinking of going with a 9" pitch 3 blade. I figure that alone should getv me close to the 4k Rpm range. Still 1500 rpm low, but from what I've been seeing it looks like that would rule out a spun prop hub and get me on plane better under heavy load. Or should I not go from a 12 to an 8 or 9.

Thanks for all the helpful replies. New to all this, and walking into the previous owners problems... Frustrating but necessary if I'm gonna learn this stuff. Lol

 

[Smgbad]

Here is the position the engine is at when I am in the water.

 

[Smgbad]

Here is another view

 

[Smgbad]

Here is what my transom mounting looks like

 

[WesNewell]

Your engine is mounted way too low. It needs to be raised 2-3 inches at minimum. You need to move the trim pin back probably 2-3 holes. Right now it's not anywhere close to parallel with the hull. Probably just doing that will allow it to get on plane. Right now it's plunging the bow into the water. Sorry, but I've never seen one so badly done. Not hard to see why it performs so badly.

 

[Smgbad]

I was leaning towards that being the main culperet after your last post. What all would involved in correcting this?

I din't think it could be moved up any more as the bracket is already resting on the lip of the boat.

The other side of the transom has a stainless steel plate (with the pattern you see on truck toolboxes all the time) with two clamp bolts and 2 larger bolts that are going through the hull (seen in the pics). Thinking I'd have to find some way to support the engine bracket by building up the transom with a block of wood or metal beam placed between the bracket and the top of the transom. Then drill new bolt holes, plug the old ones, retighten the clamps... But if there is a much easier way I'd love to hear it... Lol.

 

[WesNewell]

I'm not familiar with your boat or engine, but I'm assuming there are 2 engine mount bolts on the bottom and 2 clamps on the inside of the boat at the top. Forget the engine bracket.It's not what matters, except maybe to keep the engine from falling of during installation. My bracket is about 4 inches above the top of the transom. Not being able to see or measure what you have this is just a wag as to what I'd do. I'd remove the lower bolts and clamp the engine at the proper height. then I'd drill new holes for the lower bolts. And then find some way to seal up the old holes. Make sure you either use a plate or washers where needed, and be sure and use sealant in the new bolt holes. But the very first thing I'd do is move the trim pin back and try it then. You may be happy with just that. I wouldn't be, but....

 

[Smgbad]

I'm not familiar with your boat or engine, but I'm assuming there are 2 engine mount bolts on the bottom and 2 clamps on the inside of the boat at the top. Forget the engine bracket.It's not what matters, except maybe to keep the engine from falling of during installation. My bracket is about 4 inches above the top of the transom. Not being able to see or measure what you have this is just a wag as to what I'd do. I'd remove the lower bolts and clamp the engine at the proper height. then I'd drill new holes for the lower bolts. And then find some way to seal up the old holes. Make sure you either use a plate or washers where needed, and be sure and use sealant in the new bolt holes. But the very first thing I'd do is move the trim pin back and try it then. You may be happy with just that. I wouldn't be, but....

I lifted it to both the farthest out trim level and the one 2nd from farthest out (next to it). Here are the results of each, (3 pic sets). Does either look like it should work any better. Ultimately I want to raise the engine but if this small adjustment adds some of the 2000 RPMs I've been missing at WOT, then it's worth it in the meantime.

 

[Smgbad]

Second set with it on the farthest out setting.

 

[WesNewell]

The second with the pin all the way back is too much for sure. The first one, 2 holes toward the transom might be ok, but 3 holes from the back is probably better. Only way to find out is test it.

 

[Smgbad]

The second with the pin all the way back is too much for sure. The first one, 2 holes toward the transom might be ok, but 3 holes from the back is probably better. Only way to find out is test it.

Here is what it looks like at the Third position you suggested.

 

[WesNewell]

Yep. That's the one you want. Now if you can raise the engine About 2 inches So the av plate would touch the yardstick it would be as good as you can get it.

 

[Smgbad]

Yep. That's the one you want. Now if you can raise the engine About 2 inches So the av plate would touch the yardstick it would be as good as you can get it.

I'll try and go out later this week to test the theory, but do you think me having been using the engine trimmed nearly all the way in like I was might have been causing me to lose nearly 2k Rpm at WOT, or do you think it's a combination of the bad angle and something else. Thinking once I get it back in the water at the correct angle (and height eventually), I may still need to go with a lower pitch number than 12. They offer them all the way down to 8" for my engine. Thought about switching to a 4 blade 8" pitch instead of the 3 blade 12 that's on it.

 

[WesNewell]

It's a combo of the trim angle and engine height. Setup right, the boat should plane out and that will raise your rpm along with speed. Right now, with the way the trim is set, you are losing at least 20% of your engine power, Raising the engine will give you another 10% increase in speed and power and also raise the rpm some. The prop selector says you should be running a 13P prop with a 1200# load and 420# boat @ 30 mph. I would not lower the prop pitch at all. If you can't get on plane with at least 25 mph, then you have engine problems and/or a really bad hull problem.