Cavitation

Although the C Dory is not a boat for waterskiing or tubing, it will work. I would drop the prop down 2 inches in pitch--you will gain a better hole shot and still have a speed of about 20 to 22 mph for water skiing. The boats begin to plane at a low speed--somewhere about 10 knots--but I water skier will not get up on the ski until the boat is in the 15 to 18 mph range. If you are having problems getting the wife up on a single ski--then go up on two and drop one. The boat should be able to pull a single skier if kept light.

The weight and HP is not far off of what classic inboard ski boats were when I was first waterskiing in the early 50's. Outboards we skied behind 25 to 35 hp with single's. Later I pulled my kids up with 9.9 hp behind an inflatable---and with 25 it was planty to pull me at 180 lbs.

A lot has to do with technique of the skier and how they arrange their body and legs--straight arms etc when first taking the pull of the ski line.
 
A lot of good info. Our wet water activities are done till next summer but a spare wheel that could be put on in the summer to pull her and the kids would be good. The boat will primarily be used for fishing the Columbia river and some family adventure. I guess she'll need a second ski also. Maybe for Christmas. Thank you!
 
Does anyone else have this problem? I am experiencing engine revving on my 16 foot Angler (1989) with a new 50 HP Honda. The engine and prop were professionally installed by a dealer in Portland OR (who sells C-Dorys) and the plate is at the correct level in relation to the bottom of the boat. The prop has been checked and is not slipping. The motor is mounted at the lowest point.

The motor revs when running at 4500 RPMS or above (17 knots or more) when moving with following seas, and occasionally when going into modest swells. The revving is worse when I have a second or third passenger onboard. I have tried adding a after market plate which helped somewhat.

I also sucked air (and revved up) when making turns, but the add on plate helped that situation considerably. Could this just be a fact of life for this flat hull design?
 
adoryable":zwfbs2oj said:
Does anyone else have this problem? I am experiencing engine revving on my 16 foot Angler (1989) with a new 50 HP Honda. The engine and prop were professionally installed by a dealer in Portland OR (who sells C-Dorys) and the plate is at the correct level in relation to the bottom of the boat. The prop has been checked and is not slipping. The motor is mounted at the lowest point.

The motor revs when running at 4500 RPMS or above (17 knots or more) when moving with following seas, and occasionally when going into modest swells. The revving is worse when I have a second or third passenger onboard. I have tried adding a after market plate which helped somewhat.

I also sucked air (and revved up) when making turns, but the add on plate helped that situation considerably. Could this just be a fact of life for this flat hull design?
Click to expand...

adoryable-

I think this is just as you say in the last sentence: "a fact of life for this flat hull design", though also for the short length of the hull (compared to the wave size).

The buoyancy of the flat aft hull and the short length of the boat just lifts (pitches) the motor skeg up a lot in short, steep seas, causing ventilation.

Good that you added the plate (Permatrim would be longest and best.)

Joe. :teeth :thup
 
thanks for your response.... I think the problem I have with my little C-Dory is that it is so solid, stable, and comfortable that I forget that it is a 16ft skiffI and I expect it to perform like a much bigger, heavier boat !

The after-market plate I put on the motor is small ("SE Sport 200) so I may look into your recommendation.
 
You are not alone. Our story is almost identical. We have the same boat and same motor. We bought ours in June from the original owners. Ours does the same exact thing you described. Our mechanic is stumped. We just had the lower unit replaced and it didn't make a difference. If you figure it out we would sure like to hear from you. Thanks Marty
 
Hello C-Brats,

I'm resurrecting this old thread to see if what I experienced this weekend is normal.

We had pretty heavy following seas for two days straight and I kept hearing what sounded like my motor trying to cavitate. It was only for a split second each time but was pretty constant. Following seas are tough because they make the motor work so much harder and it sounds different from one moment to the next but it definitely sounded like it was borderline cavitating. I didn't notice any obvious slipping but again, it was tough to tell in these seas because everything is constantly changing anyways. I didn't see any sudden increases in the RPM's.

This was our first time in following seas this large, the boat had much more weight in the bow then normal, and the tabs were trimmed all the way up to try to keep the bow up. I had the motor trimmed all the way down though because of what I thought was cavitation.

This is the first time I've had this issue and I'm hoping it's related to the seas or extra weight. If I get a calm day this week to take her out and see if I notice anything.

Is this normal based on the different conditions of the boat and seas? Or is it something to be concerned about? We have another trip planned this weekend so I want to make sure we're good to go. Anything I can check for myself (she's in a slip, not on a trailer.)

Thanks
 
Above Joe mentions "Ventilation", instead of Cavitation. This is important, because the "cure" is radically different. Note that the video also explains why the boat "blows out" or ventilates in hard turns.

PROPELLER CAVITATION


Cavitation if the formation of air bubbles on the propeller. Cavitaion usually occurs as a result of damage and you'll feel it in the form of vibrations.

Cavitation is bad for three reasons. First it can physically damage the propeller. Secondly, it causes those vibrations which makes your ride less comfortable. Thirdly it reduces bite, which reduces efficiency.


PROPELLER VENTILATION


Ventilation, on the other hand, is the propeller sucking in air. This may happen because you have the engine trimmed up too high. It can occur because you're jumping waves and the back of the boat is actually coming up near the surface of the water, or perhaps you have a really sharp turn and as the boat leans your propeller is too close to the surface.

Ventilation is also bad mostly because it can cause a radical reduction in thrust and also it can cause your engine to over-rev.


CAVITATION AND VENTILATION HOW CAN YOU TELL THE DIFFERENCE?


If you can feel vibration and your propeller has visible pitting, you have cavitation and you probably need professional help. If, however, your engine is over-revving you have a ventilation issue and this is usually something you can fix yourself.
Click to expand...

Here is a link to the video which explains it very well

In Colby's case there may be ventilation, if the prop is nearing the surface of the water. Trimming the motor all of the way down may push the bow down, and actually make the ventilation worse. You want the speed in heavy seas from aft to just keep you on the back of a wave. If you want to go faster, then it is time for "Throttle man".....in offshore racing boats there is one "driver" who tends to the throttles. I drive with on hand on the throttle and the other on the wheel in heavy seas. As you come off the top of a wave, you pull the throttle back, and then apply as you start to go up the next wave.

I don't remember if you 25 has a Permatrim? It should be running just on the surface of the water under normal calm conditions. Same for "cavitation" or ventilation plate.

How high are the RPM going when you are experiencing what may be ventilation?
 
I believe you're correct, it sounds more like ventilation not cavitation.

We have a 22 with a 90 Honda. No permatrim. We've only had her about 5 months but we've used her a ton and haven't had any problems. No issues with sharp turns either.

This weekend was the first I noticed it, and it wasn't bad, it just wasn't a sound I'm used to hearing. I figured it was due to more weight in the bow, the way she was trimmed, and the heavy following seas.

Your suggestion on motor trim makes sense. I trimmed it all the way down thinking I was keeping the prop in the water but maybe I was actually lifting the stern out?

We were cruising around 4000 RPMs. About 16 knots with the seas pushing us, Fast enough to push up and over the waves and surf down.
 
I also have some kind of ventilation issue. I thought it would go away after I installed the permatrims, but it has not. On my boat, with twin 40's, I have to throttle up gradually. With my past boats I always go full throttle to get up on plane and then back down. But with our 22 both engines rev and I am forced to reduce power to get the props to bite. This only happens on acceleration. I don't notice this when turning, but I also don't make sharp turns. When my trim indicators at the helm are 'level' the cav plate is even with the bottom of the boat. I don't think the props are slipping because both lower units on my boat were brand new when I bought it. Should I lower the engines?
 
Where are the permatrims when the boat is at planing speed? Something in the dynamics of the motors and hull is causing the ventilation when accelerating. I generally bring the throttle up slowly--but you are correct that most boats do well with full throttle.

I would expect ventilation when on plane if it was the height of the motors. I would put the trim tabs all of the way down. What position are the engines in when you apply full throttle? Too far up or too far down may cause interaction between the prop wash from the twins.
 
Well, about skiing, I learned behind a 16' Peterborough wood hull with a 10 hp
Johnson when I was about 8 Y/O (? lbs) circa 1950. Last slalom, w/o spills, was
a few years ago about age 74. Now, ben-thee-don-dat. On to new stuff.

On chronic cavitation, always check the "X" dimension - an important motor
mount/lower unit mounting height on the transom for O/Bs and I/Os. Props
love sucking in air if too high.

Aye.
 
Did 27 miles today in mixed seas without any of the ventilation issues I had last weekend. So it appears it was a combination of the large following seas and how I had her trimmed.
 
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