Propeller depth

[ted655]

I would appreciate comments on prop depth.<br />I am converting from outboard to inboard. Because of shallow water operations, I can't use a typical under hull installation. My transom sets at a 12 degree angle. I am going to stibk the shaft out of the bottom of the transom, following that angle.<br />The boat is flat bottom and draws 9" of water. The prop is 20". At 12 deg. the tip of the prop is just out of the waterline. As the length gets longer the prop goes deeper into the water.<br /> Obviously I don't want a lot of shaft sticking out the stern. If I go very far with the shaft to get depth I will have to add a strut and cutless. That means a superstructure built onto the transom. UGH!<br />So... how deep into the water does a prop need to be? this is a 3 blade bronze prop on a 1 1/4" shaft.<br /> The boat is 24' X 8' heavy aluminium plate. It is diesel powered and used to push floating cabins to different areas of marsh and swamp. A slow mover not a skyboat. Any thoughts?

 

[brother chris]

Re: Propeller depth

As far as I know, the prop only has to be in the water about 3-4". <br />On an outboard, the prop is right below the dol-fins, about an inch or two.<br />On my 30'er, my prop comes out of the bottom of the boat, like you want to do. It is about 6-8" below the bottom of the boat.<br />I am sure there is others here who know a bit more than I do, but 3-4" sounds about right.

 

[snapperbait]

Re: Propeller depth

Ted.. <br />With the prop real close to the transom you may run into a problem when in reverse... With the thrust from the prop pushing against the transom, you may actually end up moving forward, or not moving at all while in reverse... This is a problem that sometimes occours on high performance boats that are equipped with surface drives...<br /><br />Also remember that with a single screw inboard your directional control at low speeds and backing down will likely be poor at best..<br /><br />My thoughts.... <br />#1. Smaller twin engine set up.. with twin engines, you'll be able to turn on a dime...<br /><br />#2. Modify the hull.. Fabricate tunnels that taper to flat towards the bow, and keep the props tucked in under the hull like this.<br />

<br />Adding full keels might help as well...<br /><br />Google search "surface drive"...

 

[ted655]

Re: Propeller depth

Thanks. Point taken on reverse problems. Your sketch resembles the Power vent system. Would that I could afford a DBD or other surface drive. I'm a poor swamp dweller who barely affords the fuel anymore. No, this has to be a low budget conversion/ This boat was a sled when the OB was on it. I solved the poor handling by making a "kick-up" leeboard up front on one side.<br />This boat is lashed to a floating cabin and pushes/ Handling is an art brlieve me.

 

[Jeff Quigley]

Re: Propeller depth

Sorry Ted, I have no suggestions for you. However, I would love to see a pic of your boat. I've been reading your thread and she sounds like and intersting vessel!<br /><br />Jeff

 

[ted655]

Re: Propeller depth

She is just a BIG jon boat, literally Aluminium plate, custom made by a profesional ship builder. Originaly made to fish the marshes for red fish. I'm the 3rd. owner and have a different task for her to perform.