Propeller Cavitation on Long Set Back Brackets

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Author Topic:   Propeller Cavitation on Long Set Back Brackets

Riptide23WA posted 06-27-2005 04:36 PM ET (US)
A question for you owners and pilots of boats with long engine setback brackets: I have recently aquired a 1992 23 Walkaround equipped with twins on an [Salt Shaker] engine bracket. My first time out in swells the other day, and I noticed the engines surging about 400-500 rpm as I slogged thru swells.
I'm assuming that because the engines are located a foot or two from the "fulcrum" of the boat (namely, the transom), that as the boat pitches down into a swell, the engines cavitate or or come close to lifting clear of the water. I would estimate the swells at about 2-4 feet. And since I was by myself, I could not observe the engines myself. But I could hear them, and did note the tachs reading out about 400 rpm swings (digital tachs, though and they don't update often enough for my tastes).
Is this typical for a boats with engines on long setback brackets? I was looking forward to this boat, but this facet has kind of put me off the bracket idea a little.
Any insight would be appreciated!
Pat
Stonington, CT
Peter posted 06-27-2005 05:12 PM ET (US)
Yes, that is typical. What propellers are your motors turning?
I have found that the 3 blade props tend to break loose fairly easily. 4 blade props have a noticibly better grip in those conditions.
Riptide23WA posted 06-27-2005 07:08 PM ET (US)
Hmmnnn, I hadn't thought about the props themselves. I think they are 13 1/2 X 17, three blade aluminum, standard Yamaha issue.
If I ever repower, I'll probably opt for 30 inch shafts, if they're available in counter-rotation pairs. The extra length might make a difference...o
Peter posted 06-27-2005 07:39 PM ET (US)
Stay with 25 inch shafts. The Yamaha props like to run deep and I'd say they are a little on the small side diameter wise.
Try running a good pair of 17P 4 blade propellers. I think you'll see a huge difference in traction mainly coming from the fact that you have more blade surface in the water at any given time.
Riptide23WA posted 06-27-2005 08:15 PM ET (US)
Alright. Guess I better comb over all the "proper prop" posts to figure out where to start.
Thanks Peter.m
jimh posted 06-27-2005 08:49 PM ET (US)
[The name of the boat model and the engine bracket have been changed to protect the innocent. Cf.: http://continuouswave.com/ubb/Forum3/HTML/006286.html ]
jimh posted 06-27-2005 08:53 PM ET (US)
I would give the boat and the long setback bracket more time to show you how it runs. I bet there will be other sea states where you will be smiling because of that bracket.
I would guess that the center of pitch resistance would be several feet forward of the transom. This means the engines are more like five feet behind the pivot point when the hull is pitching in waves.
The buoyancy of the engine setback bracket will help keep those same engines from getting dunked in the water when the boat is pitching up and down in swells.
Riptide23WA posted 06-28-2005 07:08 PM ET (US)
Hey jimh, thanks for the link. I tried a search, but didn't turn up that one. Amazing how many times a path has already been traveled.
I will admit some ignorance the other day... The boat is equipped with the Bennett trim tabs, with dual switches. Trouble is, the white lettering has worn off the switchplate, indicating how the switches operate. it's my first boat with tabs. So, without really looking, I assumed that moving the switch up raises the tabs, which is waht I thought I did, so I could just play with the engine trim alone. In fact, looking at the plate last night, the switch is labeled as to the "effect" the tabs have on the boat, so that moving the switch "up" drops the tabs, forcing the bow DOWN. So when I was out in them swells, the tabs were probably all the way down, and forcing the bow down with each swell...
Don't be scared, but my day job is a nuclear reactor operator. Good thing everything is labeled ;-)
n
WSTEFFENS posted 06-28-2005 07:53 PM ET (US)
Rip:
I don't think it is the bracket. I had a 22' Revenge WT with twin 115 hp Omc engines. It was a cut out transome, but if you didn't tuck the engines "in" it would blow out or cavitate on hard accelleration. The engines would litterally suck the water out from the back of the boat!

If you can do so, you might investigate how far the cavatation plate is below the water line. You might want to lower your motors. Or just remember to adjust the T&T before you blast off!
Later
WLS
Riptide23WA posted 06-29-2005 04:34 AM ET (US)
Yeah, I normally trim all the way in before getting up on plane. The more I think about it, the more I think my mistakes with the trim tabs were to blame. But I am going to try out each suggestion. Thanks.t

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Author	Topic: Propeller Cavitation on Long Set Back Brackets
Riptide23WA	posted 06-27-2005 04:36 PM ET (US) A question for you owners and pilots of boats with long engine setback brackets: I have recently aquired a 1992 23 Walkaround equipped with twins on an [Salt Shaker] engine bracket. My first time out in swells the other day, and I noticed the engines surging about 400-500 rpm as I slogged thru swells. I'm assuming that because the engines are located a foot or two from the "fulcrum" of the boat (namely, the transom), that as the boat pitches down into a swell, the engines cavitate or or come close to lifting clear of the water. I would estimate the swells at about 2-4 feet. And since I was by myself, I could not observe the engines myself. But I could hear them, and did note the tachs reading out about 400 rpm swings (digital tachs, though and they don't update often enough for my tastes). Is this typical for a boats with engines on long setback brackets? I was looking forward to this boat, but this facet has kind of put me off the bracket idea a little. Any insight would be appreciated! Pat Stonington, CT
Peter	posted 06-27-2005 05:12 PM ET (US) Yes, that is typical. What propellers are your motors turning? I have found that the 3 blade props tend to break loose fairly easily. 4 blade props have a noticibly better grip in those conditions.
Riptide23WA	posted 06-27-2005 07:08 PM ET (US) Hmmnnn, I hadn't thought about the props themselves. I think they are 13 1/2 X 17, three blade aluminum, standard Yamaha issue. If I ever repower, I'll probably opt for 30 inch shafts, if they're available in counter-rotation pairs. The extra length might make a difference...o
Peter	posted 06-27-2005 07:39 PM ET (US) Stay with 25 inch shafts. The Yamaha props like to run deep and I'd say they are a little on the small side diameter wise. Try running a good pair of 17P 4 blade propellers. I think you'll see a huge difference in traction mainly coming from the fact that you have more blade surface in the water at any given time.
Riptide23WA	posted 06-27-2005 08:15 PM ET (US) Alright. Guess I better comb over all the "proper prop" posts to figure out where to start. Thanks Peter.m
jimh	posted 06-27-2005 08:49 PM ET (US) [The name of the boat model and the engine bracket have been changed to protect the innocent. Cf.: http://continuouswave.com/ubb/Forum3/HTML/006286.html ]
jimh	posted 06-27-2005 08:53 PM ET (US) I would give the boat and the long setback bracket more time to show you how it runs. I bet there will be other sea states where you will be smiling because of that bracket. I would guess that the center of pitch resistance would be several feet forward of the transom. This means the engines are more like five feet behind the pivot point when the hull is pitching in waves. The buoyancy of the engine setback bracket will help keep those same engines from getting dunked in the water when the boat is pitching up and down in swells.
Riptide23WA	posted 06-28-2005 07:08 PM ET (US) Hey jimh, thanks for the link. I tried a search, but didn't turn up that one. Amazing how many times a path has already been traveled. I will admit some ignorance the other day... The boat is equipped with the Bennett trim tabs, with dual switches. Trouble is, the white lettering has worn off the switchplate, indicating how the switches operate. it's my first boat with tabs. So, without really looking, I assumed that moving the switch up raises the tabs, which is waht I thought I did, so I could just play with the engine trim alone. In fact, looking at the plate last night, the switch is labeled as to the "effect" the tabs have on the boat, so that moving the switch "up" drops the tabs, forcing the bow DOWN. So when I was out in them swells, the tabs were probably all the way down, and forcing the bow down with each swell... Don't be scared, but my day job is a nuclear reactor operator. Good thing everything is labeled ;-) n
WSTEFFENS	posted 06-28-2005 07:53 PM ET (US) Rip: I don't think it is the bracket. I had a 22' Revenge WT with twin 115 hp Omc engines. It was a cut out transome, but if you didn't tuck the engines "in" it would blow out or cavitate on hard accelleration. The engines would litterally suck the water out from the back of the boat! If you can do so, you might investigate how far the cavatation plate is below the water line. You might want to lower your motors. Or just remember to adjust the T&T before you blast off! Later WLS
Riptide23WA	posted 06-29-2005 04:34 AM ET (US) Yeah, I normally trim all the way in before getting up on plane. The more I think about it, the more I think my mistakes with the trim tabs were to blame. But I am going to try out each suggestion. Thanks.t