Inboard Diesel Prop Choices

[Dug]

I just purchased an 85 Cal 24.

Inboard Diesel 8hp 1 cyl. -
Runs fine, but only making 0.4 kts against tide. Thinking this is not normal.
Any recommendations for prop size on these boats?

[skipmac]

0.4 kts through the water or over the ground? How are you getting this speed measurement?

[bob_77903]

Prop clean ?

[Dug]

Sorry, should have been more specific....
GPS specified 0.4 knots

[Dug]

bob - don't know... having it hauled out next week. Was expecting a little drag but not that much.

[skipmac]

Originally Posted by Dug

Sorry, should have been more specific....
GPS specified 0.4 knots

Speed reported by your GPS is meaningless when trying to evaluate the prop efficiency. The GPS only shows your speed over the ground or said in another way, how fast your position moves from one place to another on the planet.

Your boat and propeller only know how fast they are going through the water. So if the water is going 5 kts due to tide or current in the opposite direction from your course and your propeller is moving the boat at 5 kts in the water, then your GPS will show you are not moving at all. The opposite, if the tide is moving at 5 kts in the same direction as the course of your boat then your GPS will (correctly) show that you are moving at 10 kts.

So to judge how well your boat/propeller is working you need to know what speed and direction the current is moving the water around your boat. Perhaps easier, find a spot where there is no current and measure it there. If that is not possible then make a run in one direction the reverse course and make the same run in the opposite direction. Compare the speeds.

[Dug]

I see your point-
In calm water I get around 4.5 knots over land.

What I guess I'm looking for would be a known solution for what seems to be poor performance in strong current conditions. Thinking the prop might be dated. It was shown to be last replaced in 06...

Otherwise, this is a characteristic I'll just have to deal with?

[zeehag]

you need tachometer measurements and speed over ground measurements and speak with your diesel man.

[Sailmonkey]

4.5 knots is about all you'll get out of less than 24' of waterline anyway!!

[Dug]

@monkey - Thanks!!

[skipmac]

Originally Posted by Dug

I see your point-
In calm water I get around 4.5 knots over land.

Like Sailmonkey said, 4.5 kts through the water is about all you can expect from a boat your size.

Originally Posted by Dug

What I guess I'm looking for would be a known solution for what seems to be poor performance in strong current conditions. Thinking the prop might be dated. It was shown to be last replaced in 06...?

Again, if you are making 4.5 kts in calm water changing prop or engine will not help. You are limited by the physics of a displacement hull boat as to how fast you can go through the water. The only thing you can do to change that is to have a boat with 150 HP or so and a high speed prop so you plane on top of the water like a ski boat.

Originally Posted by Dug

Otherwise, this is a characteristic I'll just have to deal with?

Yes absolutely. In fact if you motor through a spot with current going 6 kts in the wrong direction your GPS will correctly show you are moving backwards. But your boat will still be moving 4.5 kts through the water.

Bottom line, the boat speed is limited by the speed through the water. If you boat in an area with a strong current the only thing you can do is time when you pass through that area.

[roverhi]

As others have said, tide effects speed over the ground but not the speed through the water. A dirty, even a slightly fouled, prop seriously effects its performance. Keep that bugger clean as a whistle if you want maximum efficiency. Depending on conditions in your area, that could mean having a diver clean the prop as often as every two weeks if fouling is severe. The way things grow here, think daily cleaning wouldn't be a waste. Fortunately, the water is almost warm enough to swim in.

[Dug]

Skipmac-
I had a San Juan 23 with a 6hp outboard and never got less than 1.5 knots over land. Maybe because it was a swingkeel?
Was thinking something was wrong with my current stock inboard setup and was hoping to discover any known problems/solutions.

This confirms my suspicions. Thanks for your time-

[Dug]

Originally Posted by roverhi

As others have said, tide effects speed over the ground but not the speed through the water. A dirty, even a slightly fouled, prop seriously effects its performance. Keep that bugger clean as a whistle if you want maximum efficiency. Depending on conditions in your area, that could mean having a diver clean the prop as often as every two weeks if fouling is severe. The way things grow here, think daily cleaning wouldn't be a waste. Fortunately, the water is almost warm enough to swim in.

Thanks Roverhi- I'm sure it'd help if it were cleaned.

[jeremiason]

Dug you have been given good advice from the other members...

The simple explanation is you Can't fight length of water line... You boat's overall length is 24', but its water line is 20'. Theorectical Hull Speed won't change unless you increase the water line.


Since the prop was replaced in 2006, make sure it was the proper pitch for your engine/transmission. If it is, the only other thing you can do is keep your bottom and prop and shaft clean... You can lose over a knot of boat speed with a dirty bottom or towing a dinghy.

My first keel boat was a Catalina 27, I was in San Francisco Bay and most times had stiff 20 knot breezes throughout the summer months, directly into the mouth of my marina. It had a 6 hp Pettit Deisel Engine, which made about 5 kts in calm conditions. The only way I could get out of the marina at a reasonable speed during the summer was to motor with full sails up.

I ended up replacing the engine later on, after the orginal gave up the ghost. The new engine was 14 hp and I re-pitched the two-blade prop to accomodate the higher horsepower/transmission ratio. I got about 3/4 of a knot more speed and burned a little more more fuel.

So with all that said, if you want more boat spend, don't speed a lot of money trying to squeeze a little more speed of your boat.... Play with this one, get more experience, save your money and in a couple of years buy a bigger boat!

Fair winds!