whiskeybravo Posted July 29, 2022 Report Share Posted July 29, 2022 Shaft Length? I know, I know... this subject has prob been beaten to death. However, all of my searches on this forum, with every imaginable arrangement of search terms, just haven't given me much. Certainly not enough confidence to order a 18-20k motor. Current (blown) engine Yamaha 150 Vmax ox66, short shaft. I've found a couple of threads where people have said 20", but not with enough discussion to convince me. Two main reasons I am skeptical: 1) Performance of my current engine terrible. When I bought the boat, she would ventilate in the slightest of turns. Forget running through a winding creek. I was able to get this condition reasonably manageable by adding jackplate and going to a 4-blade. Biggest prob that was never abated was reverse thrust- never seen anything like it. Absolutely no back down thrust. Strongest reverse just clutch in- more throttle just more froth and bubbles. Wouldn't back off a beach barely touching, have to back your truck in the water to get off a trailer, come in a little hot to a dock and your gonna hit it. I always thought it was the short shaft in combination with that little pseudo tunnel or sea chest like block-out at the base of the transom maybe directing thrust down, but jacked all the way down and trimming motor up some had zero effect. Mechanic buddies baffled; told me that if it's going in reverse and spinning, it's doing what it's supposed to. Could it be just that particular motor, that the exhaust was just feeding the prop a big air bubble to spin in? If so, why don't they all do that? Also, surely there are/were plenty of that combination boat/motor out there- can't find anybody else describing same reverse problem... 2) Everything I read says transom height dictates shaft length. If I'm measuring right, I'm getting neighborhood 24.5" transom. That means 25" shaft, right? Reason I say if I'm measuring right: I'm measuring as if the "tunnel" weren't there- run a straight edge on keel back to transom and measure from top of transom to straight edge. Sorry for the novel, any insight much appreciated. Quote Link to comment Share on other sites More sharing options...
Chief5130 Posted July 30, 2022 Report Share Posted July 30, 2022 I have the same boat. I ran the same engine you have for 18 years. The first prop I had did all of the bad habits that you mention. When I switched to a Power Tech cubed and ported, it never slipped again. So I am sure that the issues you are having are due to the prop not the shaft length. About 18 months ago I re-powered with a 200 HP Suzuki with a 20" shaft. Still working on getting it proped right. Quote Link to comment Share on other sites More sharing options...
whiskeybravo Posted May 17, 2023 Author Report Share Posted May 17, 2023 Hey Chief, you get prop dialed in? if so what did you settle on? Just repowered with a suzuki 175 and ready to try props. Thx Quote Link to comment Share on other sites More sharing options...
fin-addict Posted May 17, 2023 Report Share Posted May 17, 2023 Ran my 03 Pathy 2200v with 150 vmax hdpi for 11 years with no problem even in hard turns using a 3 blade 14 1/4 x 20 prop. Max was 5600 @ 43.2 mph loaded with t top tower. Now sold. Would repower with 200 Zuke and 20” shaft. That what’s the boat calls for. Not 24” no way. 200hp with my boat would do low 50’s, my brothers did with tower. Best of luck. 👍😀⚓️fin Quote Link to comment Share on other sites More sharing options...
Chief5130 Posted May 18, 2023 Report Share Posted May 18, 2023 9 hours ago, whiskeybravo said: Hey Chief, you get prop dialed in? if so what did you settle on? Just repowered with a suzuki 175 and ready to try props. Thx I was running a Suzuki 3 x 16 x 24.5 but only got 5550 RPM at 47 - 48 MPH WOT so I went to a 3 x 16 x 23. Now I get 5850 WOT at 49 - 51 MPH 1 Quote Link to comment Share on other sites More sharing options...
q0rban Posted August 4, 2023 Report Share Posted August 4, 2023 Quote Biggest prob that was never abated was reverse thrust- never seen anything like it. Absolutely no back down thrust. Strongest reverse just clutch in- more throttle just more froth and bubbles. Wouldn't back off a beach barely touching, have to back your truck in the water to get off a trailer, come in a little hot to a dock and your gonna hit it. I suspect the sea chest causes this issue. Reverse flow hits the aft wall of the sea chest and then reflects back in the opposite direction. If it's an issue with the hull design that you tried to fix with shaft length, that would have significant negative performance impact when on plane. I wonder if the newer molds have solved this issue at all. Quote Link to comment Share on other sites More sharing options...
Chief5130 Posted August 4, 2023 Report Share Posted August 4, 2023 1 hour ago, q0rban said: I suspect the sea chest causes this issue. Reverse flow hits the aft wall of the sea chest and then reflects back in the opposite direction. If it's an issue with the hull design that you tried to fix with shaft length, that would have significant negative performance impact when on plane. I wonder if the newer molds have solved this issue at all. I have found that to get better reverse thrust, I need to trim the motor up a little. If the motor is trimmed down, the thrust hits the pocket and reduces the effectiveness. 1 Quote Link to comment Share on other sites More sharing options...
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