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Exhaust when using over hubs props http://www.hpbc.ca/viewtopic.php?f=19&t=3100 |
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Author: | allstock [ October 5th, 2015, 7:01 pm ] |
Post subject: | Re: Exhaust when using over hubs props |
Relieving the exhaust above the cavitation plate would definitely help with an over-hub prop as far as getting on plane. As long as a much louder exhaust isn't a problem. |
Author: | phillnjack [ October 5th, 2015, 7:46 pm ] |
Post subject: | Re: Exhaust when using over hubs props |
Allstock Loudness is not a problem. have you had a similar thing happen and have better results with exhaust relief ? |
Author: | AnthonySS [ October 5th, 2015, 9:22 pm ] |
Post subject: | Re: Exhaust when using over hubs props |
Tell us more about the boat - or did I miss something? |
Author: | Culprit [ October 6th, 2015, 10:38 am ] |
Post subject: | Re: Exhaust when using over hubs props |
nobody here is going to nail your set up perfect out of the gate. You have to experiment. if the slip at top end is high the motor is likely too high IMO. With T-boats we see a difference in slip with as little as a quarter inch in jack plate height, especially once you get near the high end depending on the prop you are using. You said that prop hooked up great on the transom but didnt improve top end. It didn't hook up when it was up high, either for take off or top end. So try in between, get a result, record the top end slip number. If you get under 10% i say you are getting close. once you get there try some smaller adjustments up and down to try and hone in on the perfect height. Then if the holeshot still cavitates too much i would consider exhaust relief up top to try and limit the accel slip, or try a different prop. Have you considered a bottom mod to try and make a flat pad on it? |
Author: | phillnjack [ October 6th, 2015, 5:31 pm ] |
Post subject: | Re: Exhaust when using over hubs props |
the top end was not brilliant but I did not expect it to be. I know I need more power to get the prop spinning up a bit faster, for this I am getting the bigger carbs to get another 10hp and hopefully more rpm's. with the engine jacked up I only saw an increase of 2mph over it being down deep. But I am think with more revs the slip figures will start to drop off rapidly (just my theory on that). its the pull away I was so baffled about, at first I thought the hub had spun or a plastic bag caught around the prop, this is the reason I asked about the over hub and the exhaust relief. Over hub props are something I have not had on any engine that was a thru-hub exhaust before. getting answers about this type of prop and how they like to run is not easy. all I ever hear is " jack the engine up as high as it will go " !!! But at last I am getting some feedback on this site and some sensible answers to what might be a simple problem to those who know. I did think about a pad on the bottom, but this would be adding even more weight to an already heavy boat. Also I need a lot more bow lift not stern lift, the back comes up easy once it gets going, then the front drops a bit too much for what I want to be honest. I think if I could get more bow lift and a bit less boat touching the water then speed might increase. again the reason for a jack plate with a touch of setback !!! I hope all this makes sense ha ha . |
Author: | phillnjack [ October 6th, 2015, 5:37 pm ] |
Post subject: | Re: Exhaust when using over hubs props |
By the way when using a sst 13x19 prop straight on the transom with engine up just 1 hole the slip figure drops to just 4% or 5 %. 5900rpm , 2.42-1 gear ratio 19 pitch = 42.3mph . I think that is good for what it is, but I need more speed and to get the bow up. so I thought about the over hub having so much more pitch might be the answer ? |
Author: | dubber [ October 7th, 2015, 11:38 pm ] |
Post subject: | Re: Exhaust when using over hubs props |
What were these hulls orginally rated for power out of curiosity? |
Author: | phillnjack [ October 8th, 2015, 9:52 am ] |
Post subject: | Re: Exhaust when using over hubs props |
when new/original the boat was rated for a maximum 60hp. mainly see these older ones with 2 cylinder 50 omc or 4cyl merc 50's. Now the makers rate the new boats for 50hp 2 or 4 stroke. the company who build them will sell with a Honda a Yamaha or a mercury/mariner 50hp 4 stroke. |
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