View Full Version : propellor rake and running
03-10-2010, 05:42 PM
i have a 24x26 nibril 4 blade wheel on my boat, and at cruising speed and WOT i shoot a rooster tail, and i would prefer my bow ride a little higher.. i was wondering if the rooster tail is wasted propulsion and if changing the rake would fix this at all.. also i have 1/2 by 5 foot wedge in the stern.. shes running 2550 RPM at WOT which is what my engine manual states it should run, and im running a 2.5:1 twin disc gear.
03-10-2010, 06:57 PM
Is your boat a skeg or built-down?
More rake generally results in diminished rooster tail and increased bow lift as you say--for outboards and stern drives. I'm not so sure the same applies to inboards--at least not with as much effect seen. I'd talk to a good prop guy--Michigan Wheel's tech, actually, have always steered me right over the years--but I'm not trying to push them--as changing one charachteristic may have other ramifications. Like changing RPM at WOT and cruise.
03-10-2010, 07:08 PM
its a built down hull.. im gonna shave the wedges in the stern to acheive lift in the bow.. its a displacement hull so it doesnt ride on plane.. its just in rough seas i have to run at a higher rpm, its a much smoother more comfortable ride due to the lift.. id love to squeeze a few more knots out of her, but shes a slow hull, id have to change my gearing and give her more power..
looking at the wake of the boat and seeing the rooster tail, all i can think is that im wasting propulsion
03-10-2010, 07:15 PM
actually i think she me may be a skeg hull...
03-10-2010, 08:01 PM
Yeah, it is "wasted" propulsion in once sense. In another its helping create the trim the boat adopts underway. Eliminating or diminishing the rooster tail provides more forward thrust but may change the trim, and you may find efficiency/speed drops instead of increases. Then again it might give you all you want. Props are black magic.
Was the wedge original or added later? How is it fastened? If it it can be removed, maybe run the boat see what you got as a baseline. Then shave it and reinstall after a trial? Just thinking.
03-10-2010, 08:49 PM
i believe the wedge was added later.. it had a chrysler nissan engine originally until about 1979 and it cruised around 14 kts back then, now it has a volvo tmd70b and it cruises at 9 kts.. the boat rides much more comfortable at 2500 rpms due to the lift it achieves.. i would love to go faster but i dont think theres much i can do to get that.. as far as comfort i need to get more lift at 21-2200 rpms so i need to get rid of some or all of the wedge..
Losing the trim wedge will possibly help your fuel consumption, if not your cruising speed. Won't help the rooster tail much, though. Judging from the photos of your hull, the boat is shaped to be pleasant in a seaway, handy about the pot buoys, and not particularly fast for the type. The wedge was probably applied by someone who thought that more horsepower was better, but found that it mostly aided in cloudwatching.
The roostertail is certainly lost propulsive power, but it may not be able to be re-harnessed. It usually is caused in a hull like yours by the (lack of) depth of the prop below the water's surface combined with the prop being a bit too close to the transom. There is a solution for this, but as you are (I believe) a commercial fisherman, it may not be available to you. Are you limited to the LOA or LWL of your vessel, and are you at that limit now?
The solution is rather simple in concept, but a bit spendy in realization. A distant relative did a bunch of research and experimentation on the phenomena a couple of decades ago and marketed a device to add speed and reduce wake (roostertail) with some success, but AFAIK he is retired or dead and his son is no longer running the boatshop in Kingston, Ontario. The idea is to extend the bottom of the hull about two feet further aft, sometimes with small wedges or wake-rails if required. On pleasure boats this was usually cleverly disguised as a swim platform. It served to extend the waterline, direct the wake, and keep the bow down.
Do have a talk with a reputable prop shop first, though. Michigan Wheel, as mentioned above, is a good place to start. No offense intended to any prop shop guys looking in here, but you don't want just the local blade-banger; you need to speak to someone with a large amount of experience with different blade configurations on Cape Island/Downeast hulls who can do predictive calculations. Maybe a new prop will help.
Sorry I can't make it all better for just pennies in just one post.
03-11-2010, 04:44 PM
dont really wanna alter the hull, i know shes not made for speed, she was actually downgraded on horsepower when repowered a few decades ago. i just looked into trim tabs.. so i might possibly be able to completely remove the wedges and use the trim tabs to trim it when i have a load of pots on to keep the bow down.. i guess im going to worry about the prop problems when i repower, ill end up needing a new wheel then anyways.. thank you guys for the input
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