HELP! My boat only goes 5 knots

Hello all,

I have done some work on my 34' CHB Tri-Cabin and now she can't get out of her own way. When I bought the boat in Ventura Ca, sailed her down to MDR, Ca I was able to maintain 8+ knots at 1850ish RPM. Since then I have removed the Teak decks, fiber glassed and non-skidthem. I had a diesel run-away which required a complete rebuild of the engine and the injection pump. I replaced the rotten fuel tanks with Polyethylene cells in the middle of the approximate old location, but reduced capacity by half. Repalced two group 27 batteries with 4 deep cycle tall 6 volt batteries in the same location just in front of the engine.

No other major structural changes other than significant cosmetic upgrades like wall laminate, plumbing, paint, varnish, electronics and new heads.

Boat 34' CHB Tri-Cabin
120 HP Lehman
2:1 Paragon Transmission
23" 17 Pitch LH Prop - Tuned by Wilmington Prop in Wilmington Ca

Conditions:
No Load Engine Speed - 2800 RPM
Load Engine Speed - 2050 RPM
Heavy Cavitation
Hut Tub Like white water at the stern
Top boat speed of 5.2 KPH
Easy steering to the Starboard
Incredible difficultly steering to Port
Boat has a significant plowing issue when under power

Solutions tried thus far:
Swapped props -
22/18 4 Blade - picked up .2 of a knot and reduced cavitation very slightly steering issue unchanged
24/13 3 Blade - picked up .4 of a knot and reduced cavitation more but clearly not enough, steering still not correct
23/15 3 Blade - picked up .2 of a knot and reduced cavitation very slightly steering issue unchanged

Reaching out to Trawler community for experiences, solutions or suggested boat mechanics in southern Ca area.

If you could get 8 knots before the alterations and eliminate all cosmetic ones that don't affect speed, to me that leaves the fuel tank change out. Could you have a blockage or restriction preventing enough fuel reaching the engine or is the line or filter plugged? I think you should get more than 2050 rpm WOT. Fuel??

Boatdiesel.com A few bucks to join but you will probably get better answers there.

E

If your turning the same RPM and lots of cav. my guess would be growth of bottom/prop. But you changed props and would have noticed. Maybe Tachs -knot meters lying to you, or trans is slipping? Check oil in gear case, burnt or really dirty?

I'm with La Bomba on this. I would try running the boat using a portable fuel tank hooked up to the inlet side of the on engine fuel filters just to rule out the new fuel tank and lines that could possibly be suffering from things like inadequate venting or a vacuum leak.

Boat has a significant plowing issue when under power

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Can you describe this a bit more? Is the boat floating on her designed waterline at the dock?

And if you tie the boat to the dock, put it in forward and rev the engine, what RPM can you reach?

Scott Welch
Island Eagle

Island Eagle,

She sits at a similar water line to a another CHB in my marina. And when I throttle up in the slip same issue, 2800 RPM no load and only 2050 load...

patzfan4eva said:

She sits at a similar water line to a another CHB in my marina. And when I throttle up in the slip same issue, 2800 RPM no load and only 2050 load...

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So what exactly do you mean by "plowing"?

Scott Welch
Island Eagle

The tail raises and the nose buries...

Wow, you've given us a good one.
You're turning the prop faster (2050 RPM vs 1850 RPM) and the boat lost 3 knots.
Air getting to the prop, (ventilation) or cavitation (little vaccuum bubbles forming on the surface of the prop) could cause this. No idea why this would happen on your boat.

The boat is plowing as you go faster when displacement boats usually squat as they go faster. Could the stern be raising high enough to let air to the prop?

The boat doesn't want to turn left. That will keep you out of Nascar.

Badly adjusted trim tabs could cause all of this but I'd bet you don't have trim tabs on that boat.
I think I'd be looking for a trim problem at this point.
When you figure it out, please let us know.

Did you change props or gearbox?

Could it be a LH prop running on a RH forward drive transmission in reverse?

Just a WAG.

I've toying with the idea of the wrong direction prop on the shaft, but its a counterclockwise (the aft) spinning shaft with a LH prop.

patzfan4eva said:

I've toying with the idea of the wrong direction prop on the shaft, but its a counterclockwise (the aft) spinning shaft with a LH prop.

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I thought of that as well. However, it does not explain the low RPM, in fact the RPM should be higher.

I would focus on the RPM issue first. Eliminate the fuel system as a variable, and just do it all at the dock. You should be able to achieve rated RPMs.

Scott Welch
Island Eagle

Although it shouldn't affect RPM, have you moved CG forward with your new fuel tank install?

At what RPM did you achieve 7-8 knots before the changes?

Around 1850 I was able to reach 8+ knots prior to change.

If the loss of speed and steering problems started at the same time, my guess is something on the hull dragging on the starboard side. It would be worth a quick dive to check it out. Weed growth, hull damage and delamination, a giant blister opening up, dragging a net?

In your original description you mention an over speed and rebuild. Do you think you actually getting the same power as before? Perhaps injector pump timing?...? However this would have nothing to do with the steering problem and the hot tub water.

Bow thruster? And if so something caught in the tube?

Who did the rebuild? Is the timing set correct? Giant squid attached to hull? As someone already said, join Boatdiesel. http://boatdiesel.com/index.cfm?FM=1 or call the folks at American Diesel. Good luck!

This is a weird one. It is essentially the same vessel as mine, but in aft cabin form, and yes with clean bottom I can get 8 kn at about 1850rpm, and as all he has done to the boat is cosmetic and inside the vessel, it should not have altered running charactieristics at all, even a dirty hull would not cause anything but a slower speed. I am intrigued by this diesel runaway and the engine rebuild following that. We just had a discussion on another thread about how unlikly that is, and if it blew the engine, one wonders what was done and why, and was it done right..? The new tanks and less fuel load should hardly affect the trim enough to cause ploughing and cavitation, but sound slike it might have, and there is clearly soemthing separately wrong with the steering I would say...
I also wonder like someone else mentioned re slipping in the gearbox (transmission), especially as the rev's drops off a lot under load, but the speed is way down. I also agree fuel starvation could contribute to some of the problem, but not all, especially not the trim/cavitation issue.
It will be very interesting to find out what it is in the end.

You`ve tried 3 different props,you would have noticed any abnormal underwater condition, surely that`s covered. The cavitation and white water suggests the prop is getting well spun. There could be more than one issue operating, but essentially you`d go back to what changed, and how that could affect things. What do the people who did the engine work say? Did they hit the wall, thus this understandable appeal? Were the fuel tanks redone same time as engine, or before or after? You might carry less fuel but location is unchanged. The weight of teak removed should be balanced with new glass decking.
I hope someone can help. Did anything change with the gearbox or prop shaft? That steering/white water issue sounds like prop involvement, but you`ve been there. What about a second mechanical consult by someone fresh?

As a single data point we have two FL1202 in our boat. The props are 4-blade, 23" diameter with a 16" pitch. At 1650 rpm the boat does just under 8 knots.

The original props were 3-blade, 24" diameter, 18" pitch on the starboard prop, 17" pitch on the port prop. The different pitches were to compensate for the different prop speed due to the counter-rotating gear in the starboard marine gear that changed the ratio. According to a previous owner's info these props gave the boat eight knots at about 1600 rpm. Most Grand Banks boats were deliberately over-propped by the manufacturer. The original prop pitch let the engines get to about 2,200 rpm at wide open throttle.

The current props after re-pitching, tuning, and balancing lets the engines turn up to their maximum rated rpm under load which is 2,500 pm. The governed maximum rpm with no load for the FL120 is listed in the manual as 2,650 rpm.

If you are getting 2,800 rpm at no load you are over-speeding the engine. The governor could be mis-adjusted or your tachometer is off.

This would not explain your low speed or steering problem but it would seem to indicate that your injector pump, which contains the fly ball governor, is not set properly.

This is an old thread....

Just wondering if you ever got this figured out?

Basic troubleshooting 101:

If it worked fine before you messed with it, the problem is probably with what you did to it.

So, if it worked fine with a specific propeller before you messed with it, it's unlikely that the problem is with the propeller. Same for everything else.

Go back and check your work. If you can't find the problem, have someone else check your work.

suggestion get a photo timer and mark the shaft and time it. Then do the math re. gear ratio and see if what is getting to shaft and prop matches your tach readings and photo timed rpm on fly wheel. You have to narrow down to one of these causes engine power output drop-transmission issue- increase boat resistance-prop problem. running on a bucket of fuel is a good suggestion and will eliminate that problem up to point where fuel is plugged in. What I would do is find a good mechanic sometimes that saves time and money.

It might seem obvious, but are you sure your anchor is still stowed, and not hanging
below the boat? This might slow you down and cause you to plow.
Otherwise, maybe something is caught on the front of your boat below the
waterline that doesn't weigh much, but causes a good drag when moving.
Someone mentioned a net or something like that. Maybe you have more than
one problem. Fuel/engine and dragging something.
When you changed props anything dragging would have surely been
noticed. This is a puzzle.

This thread is a year old, but i read it and am really curious how it was resolved.... He just left us hanging!

bligh said:

This thread is a year old, but i read it and am really curious how it was resolved.... He just left us hanging!

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Yes, it would be nice to hear what the cause was...it sort of defied explanation in a way.

Mind you Bligh, not a lot more puzzling than why you use a sinking boat as an avatar. Captain Bligh's vessel did not sink...the crew mutinied and set him afloat in a lifeboat.

Ha ha, so I also read this a few weeks back and PM'ed him. The cause was that he had moved his new batteries too far forward and in effect pulled his prop where it didn't have enough water flow on it. Caused cavitation and that equated to no power to push the boat. He said he moved the batteries aft and all is good.

From the original post my wife and I were betting a wrong handed prop. Cavitation and not wanting to turn in one direction....

Jeff

Greetings,
In spite of it's age, interesting thread. I am "somewhat" familiar with the 34' CHB and I looked up the difference in battery weights (roughly 150 lbs.-68 kg for our southern brethren). If, as the OP stated, the batteries stayed in the same general location just in front of the engine (allowing for the potential larger footprint of the 6v. deep cycles) it's amazing to me that so little weight would have such a profound influence on performance. The space between the front of the engine and the next forward bulkhead can't be much more than 5' (1.5m) so even if the new batteries were right up against said aforementioned bulkhead it's quite surprising to me. Maybe if I move the reserve beer (only 796 cases) more amidships and away from my port side it will eliminate that slight list....