RPM issue under load

[Compass790]

Originally Posted by alaskaflyfish

With the Martec 13x9 I can only reach 2500rpm. The prop specs I have were confirmed thru 3 sources. One being Martec who did the calculation based on my boat and transmission specs, the next was a online calculator offered by a prop manufacturer in Vancouver B.C. and finally the Prop Shop in Seattle who also did the calculations for me based on a 2 blade fixed prop. Online research on other boats with the 1gm10 Yanmar state that the Martec 13x9 was a good match. I will look again since you brought this up.


So I just checked Vicprop with the boat and engine specs and they list 13.1 x 8.8, so it looks like Im over by 0.2 on the pitch.

Are you putting in the right data to prop calc? I got 11.5 " dia & 7" ( approx) pitch using data off net for Albin27. I did guess at draft excluding keel tho. maybe it's me putting in the wrong data but if not me it explains why you can only get 2500 rpm assuming the bottom is clean& your engine/gearbox is ok.
Is it 2:1 reduction ratio gearbox? Still I'd assume the prop people would know a lot more than me but it still begs the question why cant you get anywhere near your max rpm

[alaskaflyfish]

So the draft on the Vega is 3ft 10 inches and the gearbox ratio 2.62:1. I used 5.5kts as hull speed and 3400 rpm as a target. But you are correct that even if I was slightly over propped by 1 degree could that still make 1000rpm difference? If I could get anywhere near 3600 I would be OK with that. This is why I think the OP and myself have an injection pump issue. I ordered the rebuild parts for the pump today and will do a follow up after to let people know the results,fingers crossed.


oops, looks like I got the draft wrong! What is molded draft (excluding keel)? Is that from the top of the keel to the waterline? How would I find that?

[Compass790]

Originally Posted by alaskaflyfish

So the draft on the Vega is 3ft 10 inches and the gearbox ratio 2.62:1. I used 5.5kts as hull speed and 3400 rpm as a target. But you are correct that even if I was slightly over propped by 1 degree could that still make 1000rpm difference? If I could get anywhere near 3600 I would be OK with that. This is why I think the OP and myself have an injection pump issue. I ordered the rebuild parts for the pump today and will do a follow up after to let people know the results,fingers crossed.


oops, looks like I got the draft wrong! What is molded draft (excluding keel)? Is that from the top of the keel to the waterline? How would I find that?

I put in the wrong data too! Assumed you had a 2:1 reduction box. FAIL.
Hope you get the problem solved. Not too sure that it will be your injection pump tho if it sprays properly & injector pop pressure is right
I assumed moulded draft means from top of keel to waterline as well but again don't assume is safest, maybe check with vicprop. But their calcs worked out pretty close for our boat with making that draft assumption.
Good luck

[Puregravy]

Originally Posted by Compass790

Just curious, have you checked on the vicprop calculator what your prop should be Alaskafly fish.We have a YSM8 Yanmar which is only 7hp continuous at 3200 & our boat is only 1ft shorter( probably the same @ LWL )
According to vicprop calc you are over-propped too. Can you reach that 3600 rpm?

I did and with the same query of what molded draft minus keel is, have determined I'm over propped effecting my rpm by 240-560 rpm with my smaller diameter (recommended 14.2" and I have a 13") and the impact of 1.5-2° increase of pitch for every inch decrease of diameter. I'm also going to look to make sure my tach is reading correctly.

[Puregravy]

Originally Posted by Wotname

Thinking aloud for a moment

We know:
OP only ever got 2200 rpm (presumably at WOT and no smoke).
He has a 20 hp engine with max rpm of 3600.
LOA 30' and 13x12 3 bladed fixed pitch prop
Recently rpm dropped to 2000 while underway and wouldn't increase (presumably even at WOT).
It will rev higher in neutral.

We don't know the gearbox reduction ratio but are told that a suitable prop would be ~14x8.
As a data point, I had a 14x8.5 prop with 18 hp on a 31' LOA and it would reach max hp at 3600 rpm.

The absence of smoke at WOT suggest to me the prop is not way out of spec. Probably not ideal but not the only issue.
The absence of smoke suggests there is not enough fuel getting into (or out of) the engine at WOT.

Whatever caused the rpm reduction while underway is also probably causing the low max rpm (2200).

My approach is start with the easy and work towards the hard. Check engine operation after each stage and continue down the list until either max rpm improves or smoke appears at WOT. Once you can get say 2800 or better and smoke at WOT, try prop pitching etc.

Change all fuel filters
Check pick up in tank
Check air filter for obstruction or just replace
Clean exhaust mixing elbow/bend
Ensure good flow of fuel to injector pump or rig a temporary gravity feed small tank with clean fuel
Have injectors checked
Have injector pump checked.

Ideally you should be able to make max rpm at WOT with a clean prop and bottom with no smoke.

This is probably a long winded version of what alaskaflyfish has already posted (#24)

Great point about fuel pick up. Cheaper to check than a $492 cnd injector!

[Compass790]

Originally Posted by Puregravy

I did and with the same query of what molded draft minus keel is, have determined I'm over propped effecting my rpm by 240-560 rpm with my smaller diameter (recommended 14.2" and I have a 13") and the impact of 1.5-2° increase of pitch for every inch decrease of diameter. I'm also going to look to make sure my tach is reading correctly.

BTW pitch is in inches not degrees on the calculator. 12" pitch means that ( in theory only) for 1 complete turn of the prop the boat will be driven forward 12"
So did vicprop confirm that moulded draft is the measurement from from top of keel to waterline? I'm guessing vicprop is giving you the most efficient combination
Your smaller prop than recommended would help you to get higher revs. It's your pitch that is too large. You can increase the revs by either reducing pitch or diameter.
This I know from cutting down diameter & re-pitching our prop but you can talk to the bloke that's going to re pitch yours.

[Puregravy]

Originally Posted by Compass790

BTW pitch is in inches not degrees on the calculator. 12" pitch means that ( in theory only) for 1 complete turn of the prop the boat will be driven forward 12"
So did vicprop confirm that moulded draft is the measurement from from top of keel to waterline? I'm guessing vicprop is giving you the most efficient combination
Your smaller prop than recommended would help you to get higher revs. It's your pitch that is too large. You can increase the revs by either reducing pitch or diameter.
This I know from cutting down diameter & re-pitching our prop but you can talk to the bloke that's going to re pitch yours.

Let the learning curve continue! Thanks for the pitch degree connection. Vic prop did not give me feedback on moulded draft. I have inquire about this on the Catalina 30 FB group. Have not heard yet.

[Compass790]

Of course there could be a mechanical reason for your problem too as suggested by others. I just thought your pitch sounded big.
I am envious of your Desolation Sound trip

[Puregravy]

Originally Posted by Compass790

Of course there could be a mechanical reason for your problem too as suggested by others. I just thought your pitch sounded big.
I am envious of your Desolation Sound trip[emoji2]

Update:

The nipple off of the elbow although looking not too bad was plugged. I have cleared that and the elbow is ready to reinstall. The engine side however looks dodgy. I have been working to clear though it appears that the elliptical shape may be cast and not as a result of carbon build up. I am searching for pics of stock but as of yet have not found. The volvo site shows the schematic part break down from the wrong side.

I'm also adjusting the valves as there is no record in the maintenance log of it being done over its roughly 1700 hours of use.

I'm going to deal with the prop pitch at our spring haul out as the local prop guru has a week long back up and the desire to get back out is strong.

[Puregravy]

Originally Posted by Puregravy

Update:

The nipple off of the elbow although looking not too bad was plugged. I have cleared that and the elbow is ready to reinstall. The engine side however looks dodgy. I have been working to clear though it appears that the elliptical shape may be cast and not as a result of carbon build up. I am searching for pics of stock but as of yet have not found. The volvo site shows the schematic part break down from the wrong side.

I'm also adjusting the valves as there is no record in the maintenance log of it being done over its roughly 1700 hours of use.

I'm going to deal with the prop pitch at our spring haul out as the local prop guru has a week long back up and the desire to get back out is strong. Attachment 176805

Thanks again to all. Set out to adjust the valves #2 exhaust had lost its tappet(?) and had started to pound the top of the valve and wreck rocker arm. Luckily caught it before valve dropped. 6 bolts away from pulling off my first head.

Can a machine shop dip my exhaust manifold to remove unwanted deposits? Would rather clean than pay $2800+ CND for a new one...

[a64pilot]

Yes a machine shop that is going to do your head can boil out the exhaust manifold, just don’t leave any aluminum bit still attached to it, if there are any.
Sorry to hear about this, but you can fix it.
Have the injectors overhauled while your at it, and don’t let them “skim” the head if at all possible to avoid, it shouldn’t need it, but some places seem to want to do all, I guess it’s another sale?

[Compass790]

Glad ya found te smoking gun. I'm with A64 dont skim head. If you worried about face use Loctite 515 or 518. It works good, expensive but cheaper than milling.Performs well above it's temp rating. I've used it on air-cooled engines & steam lines.
I'd hold off on re-pitching until you see how it performs after you've cleaned up the mechanical issues. Good luck with repairs

[Capt Ben]

Re the exhaust mixing elbow, check that you can readily blow through it with no restrictions. I thought mine was good but after doing this test I found out otherwise. I replaced my elbow with a ss version. Less expensive than original Volvo cast one.

[Puregravy]

Originally Posted by Capt Ben

Re the exhaust mixing elbow, check that you can readily blow through it with no restrictions. I thought mine was good but after doing this test I found out otherwise. I replaced my elbow with a ss version. Less expensive than original Volvo cast one.

Thx! SS ordered and far better design that'll clear much better. Appreciate the response.

[Sirius 26]

Hello guys I'm new on this forum and been reading your posts about the same problem I've been experiencing with my engine.
Have you solved the low rpm issue???

I just bought a german 1978 sailboat sirius 26 with a 2002 diesel volvo penta md2010d (10hp 2cylinder).
I noticed someone mentioned and asked if you tried to see if rpm go higher at wot in reverse but nobody replied about reverse rpm.

When i bought my boat max rpm at wot was 2500 and the rest of the throttle lever would just go forward with no effect. When i drive the boat at wot i don't see any smoke but the back of the boat turned black where the exhaust is, so therefore it does smoke i guess. People been asking me if I've been wot a lot due to black deposit on back of my boat, I said not really cos it doesn't go over 2500rpm.

I cleaned the bottom of the boat and propeller (it wasn't bad and i doubted this was the reason for low rpm at wot) and now the boat goes max to 2800rpm forward. In neutral it revs to 3500rpm no problem. Then i tried drivin my boat in reverse wondering if wot will also be 2800 but i topped 3500rpm in revese easy maybe could rev even higher but was hard to steer in reverse that fast.

Therefore my engine tops at least 3500rpm in neutral and in reverse drive but only 2800 in worward drive. What could this problem be??? Sounds weird and in no way would a boat have better hidrodinamics in reverse than in forward, otherwise they'd build boats the other way around.
Is it a gear problem since forward has lower rpm than reverse?
Sasha