Boat Prop Protectors

vortx · Mar 6, 2021

I have designed prop protectors for a 42 ft Preformance Trawler twin engine. My problem is attaching it to the hull. my initial thought is 6x6x1/2 mounting plates inside and out bolted with 3/4" bolts. Figured more is better. Further contemplation (during morning rituals) gave me some issue with this profile. My scare is that if I hit something it will tear a 6x6 hole, or 3 or 6 (6 landing pads). I am now looking at bolts maybe in the 1/4" or 3/8 size where if it gets hit the bolts will strip out and I can ORDER the crew to stick their finger(s) in the holes so we do not sink. Naturally as in life is compromise (unless it is with your wife) so how do come up with the size to hold everything together but strip when required...or is there a better configuration/attachment to accomplish the task.
Planning a trip from Port Clinton, Ohio to Merrit Island, Fl about June 10th. Looking for someone with fingers to help me for the whole trip or any part thereof.

Milland · Mar 6, 2021

I'm not sure what you designed, but my thought is two or so ramped longitudinal fins (much like miniature keels) in front of the props from ~1/4" 316 stainless. The ramp to push debris down below the prop, the fin to minimize drag.

Actual sizing and mounting is up to you, but you have to think about the ship's design and how the strain of impact would be passed into the structure. If you allow an impact to "rip" the fin off, perhaps a few stainless cables attached to prevent loss? But then you have to make sure the cables can't foul the props.

Screw it, or more correctly, de-screw it and go to magnetohydrodynamic drive...

13engines · Mar 6, 2021

And what does this have to do with "CNC Machining" I ask?

Hardplates · Mar 6, 2021

13engines said:
And what does this have to do with "CNC Machining" I ask?

I assume it is to be "billet"

kustomizer · Mar 6, 2021

Use 1/2" allen cap screws without washers to mount them to the hull, when you rip one off hitting something you can fill the holes with wine corks so you will have the crew still able to bail, just remember the more bolts you use the more wine you will have to haul with you.

dlaw · Mar 6, 2021

Use the 3/4 bolts but neck them down to provide a shear point?

Scottl · Mar 6, 2021

kustomizer said:
Use 1/2" allen cap screws without washers to mount them to the hull, when you rip one off hitting something you can fill the holes with wine corks so you will have the crew still able to bail, just remember the more bolts you use the more wine you will have to haul with you.

Old Navy and merchant marine way was to pound wooden plugs into holes. If you've ever seen the film San Demetrio London, which is based on a true story the crew is seen pounding various size plugs into the shell holes on the deck of the burning gasoline tanker after they re-boarded her. This same technique was used to plug smaller holes in hulls on ships damaged in combat along with using heavy bracing timbers to prevent damaged hull sections from caving in.

MCritchley · Mar 6, 2021

Sailboat racing instructions dictate that a competitor must tie a wooden plug near any thru hull fitting in the event of water ingress. I’ve had to lift up the floor boards for a post race inspection after a Mackinac race, they take it serious. Not a bad idea to keep them tied to where they will be used.
No need to look through the junk drawer when your sinking.

We also carried a few spares and a larger foam plug maybe 6” dia in case we had a puncture from a log or semi submerged object.

vortx · Mar 6, 2021

Cap screws.

kustomizer said:
Use 1/2" allen cap screws without washers to mount them to the hull, when you rip one off hitting something you can fill the holes with wine corks so you will have the crew still able to bail, just remember the more bolts you use the more wine you will have to haul with you.

Now we are talking. Had not thought of cap screws. Do you think beer bottle caps and duct tape would be Ok too? Some people do not like wine so I try to be obliging.

vortx · Mar 6, 2021

MCritchley said:
Sailboat racing instructions dictate that a competitor must tie a wooden plug near any thru hull fitting in the event of water ingress. I’ve had to lift up the floor boards for a post race inspection after a Mackinac race, they take it serious. Not a bad idea to keep them tied to where they will be used.
No need to look through the junk drawer when your sinking.

We also carried a few spares and a larger foam plug maybe 6” dia in case we had a puncture from a log or semi submerged object.

Lots of ways to plug holes. Stuff the sheets off your bed in it. May be a problem when your mother finds out after you put them back on the bed. Just make sure you dry them real good.

primeholy · Mar 6, 2021

vortx said:
Now we are talking. Had not thought of cap screws. Do you think beer bottle caps and duct tape would be Ok too? Some people do not like wine so I try to be obliging.

Bourbon also comes with cork, but it is attached to a stopper

vortx · Mar 6, 2021

Milland said:
I'm not sure what you designed, but my thought is two or so ramped longitudinal fins (much like miniature keels) in front of the props from ~1/4" 316 stainless. The ramp to push debris down below the prop, the fin to minimize drag.

Actual sizing and mounting is up to you, but you have to think about the ship's design and how the strain of impact would be passed into the structure. If you allow an impact to "rip" the fin off, perhaps a few stainless cables attached to prevent loss? But then you have to make sure the cables can't foul the props.

Screw it, or more correctly, de-screw it and go to magnetohydrodynamic drive...

Attached are my thoughts so far #1 with a ring around the prop would probably improve my fuel mileage but would probably be more prone to tear off and more complicated to build. #2 I think is along the lines you are describing. Also the "V" would stand the boat up when (if) aground. Only 3 types of real sailors, people that have run aground, those that will run aground, and those that are aground. The most reliable depth finder is your keel, but I digress. I was thinking 1/4" but decided to go with 1/2" just to be sure, plus a water jet doesn't know the difference. Need to discuss with the shop I use. They may have some scrap lying around since the pieces are small.
Many thanks for your input.

kustomizer · Mar 6, 2021

vortx said:
Now we are talking. Had not thought of cap screws. Do you think beer bottle caps and duct tape would be Ok too? Some people do not like wine so I try to be obliging.

I think they would work OK but you would likely have to put them on the outside and use that new flextape

Ziggy2 · Mar 6, 2021

vortx said:
Attached are my thoughts so far #1 with a ring around the prop would probably improve my fuel mileage but would probably be more prone to tear off and more complicated to build. #2 I think is along the lines you are describing. Also the "V" would stand the boat up when (if) aground. Only 3 types of real sailors, people that have run aground, those that will run aground, and those that are aground. The most reliable depth finder is your keel, but I digress. I was thinking 1/4" but decided to go with 1/2" just to be sure, plus a water jet doesn't know the difference. Need to discuss with the shop I use. They may have some scrap lying around since the pieces are small.
Many thanks for your input.

I think you need to research your idea and concept much more than you have.

Your number #1 concept will most likely greatly reduce the performance of the props. There has been a lot of research done in this area an your design as it currently stands will reduce the prop performance. There is a lot that goes into the ring design and shape plus how the prop blades need to be changes to accommodate the ring.

The second design will also increase the drag but not affect the prop efficiency as much however this version will most certainly cause large localized hull loads if run aground. In other words, unless your significantly increase the hull strength in the struct area, you will likely rip the bottom of your boat out and be setting on the bottom before you even know it.

You have not mentioned exactly what you mean by a "performance trawler" which makes me think more of a 42ft. semi planing hull with not a lot of keel depth. There are very good reasons why your concept is not used and I think it would be very prudent for you to understand why. If you proceed without doing the research, I suspect that you will have a very high probability of your boat becoming another statistic setting on the bottom.

Milland · Mar 6, 2021

vortx said:
Attached are my thoughts so far #1 with a ring around the prop would probably improve my fuel mileage but would probably be more prone to tear off and more complicated to build. #2 I think is along the lines you are describing. Also the "V" would stand the boat up when (if) aground. Only 3 types of real sailors, people that have run aground, those that will run aground, and those that are aground. The most reliable depth finder is your keel, but I digress. I was thinking 1/4" but decided to go with 1/2" just to be sure, plus a water jet doesn't know the difference. Need to discuss with the shop I use. They may have some scrap lying around since the pieces are small.
Many thanks for your input.

I have a bit of understanding of aero/hydro dynamics, and did work in a research wind tunnel for a while, but am hesitant to give advice to anyone trying to do what you are without massive caveats.

The fins I was suggesting are just lengthwise strakes (in the airplane sense, not boat Strake (aeronautics) - Wikiwand) that would go in front of the prop, sort of like the underside fins you see on surfboards, but longer and deep enough at the trailing end (in front of the props) so that larger debris or the waterway bottom would hit before the prop did.

So not surrounding the prop, and with a long engagement surface to the boat bottom to allow many points of attachment for load spreading.

The drag penalty would be unknown without testing, and could be considerable depending on a bunch of factors.

I would not bother with either of the two designs you attached, my gut tells me too much drag, too little protection.

Again, no formal advice here, just my qwikndurty take on things.

DDoug · Mar 7, 2021

You mill the boat out of a billet, and the prop protectors are integral....duh ... :nutter:

moonlight machine · Mar 7, 2021

Why don't you carry spare props if you are so worried?

Scottl · Mar 7, 2021

moonlight machine said:
Why don't you carry spare props if you are so worried?

Because severe impact can not only damage the prop but what it is attached to. Even with shear pins drive systems can be damaged because not all the stress is in the shear plane of the pin.

Milland · Mar 7, 2021

digger doug said:
You mill the boat out of a billet, and the prop protectors are integral....duh ...

Wait - now we're starting with a giant block of 316 stainless? Or do you propose billet fiberglass?

706jim · Mar 7, 2021

I once had a student trying to machine a car wheel rim out of billet carbon fiber.

No, you didn't read that wrong, he laminated a zillion layers of carbon into a huge block and then tried to cut it to shape on our GR15 router.

And no, it never came close to completion.

Boat Prop Protectors

Plastic

Diamond

Stainless

Stainless

Diamond

Aluminum

Diamond

Stainless

Plastic

Plastic

Aluminum

Plastic

Attachments

Diamond

Stainless

Diamond

Diamond

Diamond

Diamond

Diamond

Titanium

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