Inboard Diesel Prop Choices

[skipmac]

Originally Posted by 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-

Cannot explain why you showed a higher speed over ground with the San Juan. The swing keel would have minimal effect. The maximum hull speed of a sail boat through the water is 99% determined by the length of the boat or more correctly, the length of the water line. The difference between a 23' and a 24' boat should not be over a 1 kt difference in speed.

All the recommendations, clean prop, etc can help and keep your speed through the water at the max but you cannot change the max limits.

Are you sure you measured the speed over ground in the San Juan when the current was the same as when you measure the speed in the in the Cal?

What you need to measure is the speed through the water. Doesn't the boat have a built in knotmeter? What does that say?

[Dug]

Originally Posted by skipmac

Cannot explain why you showed a higher speed over ground with the San Juan. The swing keel would have minimal effect. The maximum hull speed of a sail boat through the water is 99% determined by the length of the boat or more correctly, the length of the water line. The difference between a 23' and a 24' boat should not be over a 1 kt difference in speed.

All the recommendations, clean prop, etc can help and keep your speed through the water at the max but you cannot change the max limits.

Are you sure you measured the speed over ground in the San Juan when the current was the same as when you measure the speed in the in the Cal?

What you need to measure is the speed through the water. Doesn't the boat have a built in knotmeter? What does that say?

In all fairness, I sailed the san juan in a different location on the ICW. The knot meter on the Cal 24 had been intentinally been disconnected by the PO to use the thru hull fitting for something else (I was told). I plan on reconnecting this as it seems to have a bearing on determining motoring performance.
Also, the bottom will be cleaned when it gets hauled out next week, so I'm sure I'll make up a little headway there also-

[Dug]

Originally Posted by skipmac

Cannot explain why you showed a higher speed over ground with the San Juan. The swing keel would have minimal effect. The maximum hull speed of a sail boat through the water is 99% determined by the length of the boat or more correctly, the length of the water line. The difference between a 23' and a 24' boat should not be over a 1 kt difference in speed.

All the recommendations, clean prop, etc can help and keep your speed through the water at the max but you cannot change the max limits.

Are you sure you measured the speed over ground in the San Juan when the current was the same as when you measure the speed in the in the Cal?

What you need to measure is the speed through the water. Doesn't the boat have a built in knotmeter? What does that say?

Originally Posted by 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!

Thanks! I was looking for something bigger, but couldn't pass up this opportunity.
Didn't consider the limitations of designed hull speed until now-
If I had to guess, what I have (propulsion wise) is sufficient and I'll have to time my outtings with the tide for gettting in/out of the marina.

[skipmac]

Originally Posted by Dug

In all fairness, I sailed the san juan in a different location on the ICW. The knot meter on the Cal 24 had been intentinally been disconnected by the PO to use the thru hull fitting for something else (I was told). I plan on reconnecting this as it seems to have a bearing on determining motoring performance.

Unless you know the speed of the current the speed you see on your GPS tells you nothing, zero, nada about the boat performance and whether or not you have the right prop or a dirty prop or dirty bottom. The only thing that your boat knows is the water. The boat does not know if that water is moving or not. The boat knows how fast it moves compared to the water it is floating it, moving or not.

Comparing speed on a GPS in one place or another, or even one time or another if the current is from tide going in and out is meaningless

Originally Posted by Dug

I plan on reconnecting this as it seems to have a bearing on determining motoring performance.

The speed through the water measured by a knotmeter in the boat not only has a bearing on determining performance (motor or sail) it is the only way.

[Dug]

Originally Posted by skipmac

Unless you know the speed of the current the speed you see on your GPS tells you nothing, zero, nada about the boat performance and whether or not you have the right prop or a dirty prop or dirty bottom. The only thing that your boat knows is the water. The boat does not know if that water is moving or not. The boat knows how fast it moves compared to the water it is floating it, moving or not.

Comparing speed on a GPS in one place or another, or even one time or another if the current is from tide going in and out is meaningless




The speed through the water measured by a knotmeter in the boat not only has a bearing on determining performance (motor or sail) it is the only way.

I do understand how a treadmill of tidewater can have my boat at a standstill even if it's motoring at top speed.
But thanks for explaining all that

I guess what I'm getting at, is whether or not my boat needs any kind of performance upgrade (i.e. changing props) so I don't spend half a days light traveling 2.5 miles to go sailing. Sounds like I'm already working at optimal performance minus a cleaning/maintenance/painting.
Timing my departure and arrival with the tides will be key.

[kmacdonald]

I have a Catalina Capri 22 with a 3.5 HP Tohatsu that does 5.5 kts in calm seas but wont go into 15 or 20 kts of wind at all. Works for me since I sail when there is wind and the motor only weighs about 30 lbs.

[skipmac]

Originally Posted by Dug

I do understand how a treadmill of tidewater can have my boat at a standstill even if it's motoring at top speed.
But thanks for explaining all that

Apologize if I have been explaining something you already knew, but from your original question and responses it was not obvious to me that you had a grasp of this concept.

Originally Posted by Dug

I guess what I'm getting at, is whether or not my boat needs any kind of performance upgrade (i.e. changing props) so I don't spend half a days light traveling 2.5 miles to go sailing. Sounds like I'm already working at optimal performance minus a cleaning/maintenance/painting.
Timing my departure and arrival with the tides will be key.

Your boat might be capable of a little more speed but perhaps not with the existing engine. Do you know the formula for calculating the theoretical max hull speed for a displacement design?

[bruce smith]

After the bottom is cleaned, take her for a spin in calm, still water.
Then ,your propeller choice is based on engine speed(tach reading).
4.5 knots sounds a bit slow.
Tide is water moving up and down, it will only affect your speed if you hit bottom. (current, is your current concern here)

[Dug]

Originally Posted by bruce smith

After the bottom is cleaned, take her for a spin in calm, still water.
Then ,your propeller choice is based on engine speed(tach reading).
4.5 knots sounds a bit slow.
Tide is water moving up and down, it will only affect your speed if you hit bottom. (current, is your current concern here)

Very good point. haa! Thanks for the clarification.

[MarkSF]

I recently sailed a 85 Cal 24 with a 5hp outboard. We could get 5.2 knots at 3/4 thottle.

4.5 knots is a bit slow, I'd expect a bit more with 8hp. Your hull speed is 6 knots and you should be able to get within 0.5 knots of that.

Is the bottom clean? Is the prop clean? (makes a tremendous difference). Is the engine reaching rated RPM?

[Dug]

Originally Posted by skipmac

Apologize if I have been explaining something you already knew, but from your original question and responses it was not obvious to me that you had a grasp of this concept.



Your boat might be capable of a little more speed but perhaps not with the existing engine. Do you know the formula for calculating the theoretical max hull speed for a displacement design?

I know it exists...beyond that-? not quite the boat designer-
What I do know is this: I'm thinking the inboard should fight the current better than it does....
Maybe using the gps speed as a litmus was a misleading comparison.

If my boat is not motoring as efficient as it should be, then any improvement will yield a faster land speed at a comparable current.....right?

[Dug]

Originally Posted by MarkSF

I recently sailed a 85 Cal 24 with a 5hp outboard. We could get 5.2 knots at 3/4 thottle.

4.5 knots is a bit slow, I'd expect a bit more with 8hp. Your hull speed is 6 knots and you should be able to get within 0.5 knots of that.

This is what I was thinking?
I'm having it hauled out next week, so after that, I'm sure I'll see an improvement-
Thanks for the info on the hull speed!

[MarkSF]

The formula for size of engine required to get close to hull speed is 2hp per 1000 lbs, btw, which you have.

Let us know what speed you get after the haulout, using the two directions method.

The effect of a dirty prop (and bottom) is hard to over-emphasise, it can really kill performance.

That year of Cal 24 is a really great boat, btw. I learned to sail in one.

[Dug]

I guess it's hard to measure anything while there is an unknown.
I'll make sure my knot meter is hooked up and do just that- 180 degree runs.
Two weeks from now should tell the tale-

[MarkSF]

The 180 degree turn is for use with the GPS, not the knotmeter, the principle being that if there is current the effect will be averaged out.

For example :

Current 2 knots. Boat speed through the water 5 knots. In one direction the reading on the GPs will be 5+2 = 7 knots, in the other direction it will be 5-2 = 3 knots.

Average speed = (3+7)/2 = 5 knots.

Also using this method the tidal current can be precisely determined. It's half the difference between your two speed measurements.

The knotmeter is very unlikely to be accurate anyway, at least until you calibrate it. How do you calibrate it? Using the GPS and the above measurement technique.

Without calibration the average knotmeter could be 20% out.