Props... How high is too high???

[comstox]

OK guys, I need some brain power here from some of you smarty farty physics, engineer and propulsion specialists!
I had posted earlier about building a cool fast little river boat and was considering an inflatable, a small john, boat and the possibility of a long tail type of mud motor, but in the end I think a small boat with a 15?20 hp 2 stroke engine will meet my needs just fine.
But I have a prop height question:
How HIGH is TOO HIGH??? Keep in mind that, at times I will be in shallow water and don?t want my prop sticking way down under the boat. (Like in pic A.)
Looking at the blow pic I obviously feel that letter C. is TOO high simply because the prop is above the back of the boat, but is there some sort of scientific formula for the MAXIMUM HEIGHT? How will I know if it?s too high?
Can I lift the motor up (Even if I have to modify my transom,) so it is JUST even with the bottom? (Just a bit higher that the prop in digram B???)
Just a little background: I?m sure the engine will need to be a short height (short shaft,) of some sort. I?m shooting for a 10??12? boat.
I?ll shut-up and let you guys weigh in.

​

 

[Stumpalump]

You have to experiment. Start high at B. Ruff water likes lower. If you can't trim up high enough to get max speed then go lower. If ventilation in turns bothers you or you can't adjust trim on the fly then go lower. On most boats I've had with the cavitation plate even with the bottom it's too high to trim up for max speed. Trial and error is your freind. I just changed props and now it ventilates a little more at top end because it's going faster. I'll drop it one hole. Passengers change the way it performs as well that's why they sell powered Jack plates. You just need to find your happy place between never ventilating and ventilating only when trimmed up and making sharp turns.

 

[Scott Danforth]

best running is A

get a good GPS with up to date charts to avoid skinny water.

 

[Sea Rider]

If will be runing an inflatabe, none of the 3 apply as exact mars rocket science. In real boating world what is all about is at which lower leg height is water flow passing by at full plane. Inflatables must run trimmed to neutral at 90 deg with deck weight evenly distributed. With a maximized prop will achieve for a OB to deliver full HP out of them. Which brand OB do you have in mind to power a 10-12 ft inflatable ?

The issue with posted examples is that all are based on dry installations, it's a different wet story when boat is floating and water at speed is passing under hull when combo planes out..

Happy Boating

 

[comstox]

You just need to find your happy place between never ventilating and ventilating only when trimmed up and making sharp turns.

Stump, I understand all that you said except "ventilating." Please elaborate.

 

[comstox]

Which brand OB do you have in mind to power a 10-12 ft inflatable ?

Although I don't think it will be an inflatable, it may end up to be: Time will tell. Thanks for that info!
This is good stuff guys! Keep it coming!
Thanks again!

 

[Stumpalump]

Stump, I understand all that you said except "ventilating." Please elaborate.

Ventilation is sometimes misquoted as Cavitation. Ventilation is when the prop is so close to the surface that it sucks air. You can hear the sound and increase in RPM and feel the change in thrust. Kinda sounds and feels like the prop is slipping because that's what it does. Inflatables can also creat air bubbles that feed the prop. Basically inflatables are sluggish anyway so unless you have a specific need for one then stick to an aluminum V or flatbotttom in the 12-16' range. Performance will be superior in almost every way over an inflatable. With height you are putting the cart before the horse. You dial a boat in closer to this order. Match the prop to the engines recomended Wide Open Throttle (WOT) range. Move weight around and fool with trim settings to get it to plane and achieve max speed. Height just milks the last bit of performance out of it. Add it all up and it makes a huge difference. I bought a 28 mph boat that went 37 after I tinkered with the setup. My current boat went from 30-34 so you are on track in your quest to understand these things.

 

[jimmbo]

Once you start raising the engine, you need props that are designed to run less fully submerged. Unless your boats is going 45mph or faster, there really isn’t much reason to.if you need run in shallow water, perhaps you should look at outbound jets

 

[Faztbullet]

Get a hull with a tunnel and you can run at C !!!!! Look at SeaArks post......."1652 MVT. SeaArk's Tunnel Hulls are designed to be used with a conventional prop driven engine. The transom is higher than on a conventional hull and a ?tunnel? is designed into the bottom of the boat which allows the engine to be raised so the prop turns inside the ?tunnel"

 

[ahicks]

Check into the way a flats boat is set up. They use a jack plate that will let a flat bottom boat run on a wet rag - SLOWLY - by raising the engine up well beyond what you can get away with otherwise. There is no expectation off efficiency while raised like this. The idea is to let the boat run around in shallow water without smashing tearing up the prop/lower unit. Once lowered all the usual rules apply and you can run normally again.

 

[comstox]

stick to an aluminum V or flatbotttom in the 12-16' range. Performance will be superior in almost every way over an inflatable.

Stump, this is why I have all but abandoned the Inflatable idea. Thanks for the confirmation!

Move weight around and fool with trim settings to get it to plane and achieve max speed.

I sort of instinctively knew this was what I was going to need to do. Thanks AGAIN for the confirmation!!

Add it all up and it makes a huge difference. so you are on track in your quest to understand these things.

You guys are so helpful, you are helping me make better, wiser choices for almost every question I ask! I am very glad I stumbled upon the iBoats Forum! Thanks AGAIN!!!

 

[comstox]

Check into the way a flats boat is set up. They use a jack plate ...

Ahicks, what is a Flats Boat? and Can you show me a picture of a "Jack Plate?" Thanks!

 

[comstox]

Install a 'Jack-Plate' I think THAT is my answer! I can experiment on the go without having to bring the boat back and forth to the water every time I need\feel I need an adjustment!!! Brilliant idea!

Can anybody confirm what this guy is saying as a rule of thumb? At the 30 second mark he says, "That 'plane' (Anti cavitation/ventilation plate,) wants to be even with the keel of your boat." Is that a good rule-of-thumb?

 

[NYBo]

Install a 'Jack-Plate' I think THAT is my answer! I can experiment on the go without having to bring the boat back and forth to the water every time I need\feel I need an adjustment!!! Brilliant idea!

A jackplate would be overkill on a small tinny, and will make balancing the load more difficult by moving the motor farther back.

Can anybody confirm what this guy is saying as a rule of thumb? At the 30 second mark he says, "That 'plane' (Anti cavitation/ventilation plate,) wants to be even with the keel of your boat." Is that a good rule-of-thumb?

It's a good starting point. Some rigs do better with the motor a little higher. Ironic that he says to make sure your motor is the right length. His is too long, probably a 25" leg while the boat probably has a 20" transom, which is why he has to jump through these hoops.

 

[ahicks]

No doubt a jack plate moves the engine back, but I don't think the amount is much at all. Most (for the size engine that might be in play here) I've seen are 4" or so at most.

The bigger point, is that raising the engine up that high is really going to limit the boat's speed. You would use it like that only while fishing. It would then need to be moved back down to allow the potential to get on plane.

Flats boats (often with pretty fancy jack plates) are commonly used by many/most fishing the coastal areas of the Gulf states.

Here's a pretty neat small one on Tampa Craigslist. The stand over the engine is commonly stood on while using a pole to move the boat around for fishing in really "skinny" (shallow) water.
https://tampa.craigslist.org/hil/boa...362468589.html

Here's a nice jack plate:
https://www.amazon.com/Micro-Jack-Pl.../dp/B0053HQAPY

 

[Scott Danforth]

one thing to caution with flats boats. watch the tides. I know plenty of guys that end of getting stuck in a flats hole until the tides come back in4-6 hours later. too focused on fishing to notice the birds walking around them

 

[ahicks]

You need to have "been there, done that" or something close, to fully appreciate that warning. To me, I've spent most of my life boating, but nearly all of it's been based in the mid west until recently. Point being, tides are foreign to my natural instincts.

The incident that changed that for the most part here, happened after beaching the 'toon on a nice white sandy beach on an island maybe 5 miles from shore - without considering that the tide was coming in. I was a couple hundred yards from the boat when I looked up to see it floating several feet off shore! By time I got to it (I'm a fat old man!) it was a 100' from shore and I was swimming as fast as I could to catch up with it! Happy ending sure, but tide lesson learned here! -Al

 

[Scott Danforth]

You need to have "been there, done that" or something close, to fully appreciate that warning. To me, I've spent most of my life boating, but nearly all of it's been based in the mid west until recently. Point being, tides are foreign to my natural instincts.

The incident that changed that for the most part here, happened after beaching the 'toon on a nice white sandy beach on an island maybe 5 miles from shore - without considering that the tide was coming in. I was a couple hundred yards from the boat when I looked up to see it floating several feet off shore! By time I got to it (I'm a fat old man!) it was a 100' from shore and I was swimming as fast as I could to catch up with it! Happy ending sure, but tide lesson learned here! -Al

BTDT, own the teeshirts and the TowBoat US insurance

were you out at Eggmont? or the islands north of Dunedin? see that happen often out there. I was out at beercan and needed to wait for the tide to come back in once. then again, I was following my GPS tracks off of Bean point on AMI and the channel had moved 60' after a storm. found myself going from 19' of water to 6" while cruising at 20mph.

also see boats stuck in the flats just off the Skyway Causeway (Near Cow and Calf key) as well as the flats on the Manatee River near Robinson Preserve.

there are 3 kind of boaters in the FL ICW...... those that have run aground, those that will run aground and those that lie about it.

 

[Sea Rider]

Although I don't think it will be an inflatable, it may end up to be: Time will tell. Thanks for that info!
This is good stuff guys! Keep it coming!
Thanks again!

That applies for any type of motor/boat as well, when Top OB/Transom height is dialed, combo will work exceptionally well under most water cond. Post a pic of the lower leg shot sideways when OB is at hand to go from there..

Happy Boating

 

[ahicks]

Scott, we're up a little further north, in the Homosassa/Crystal River area (poo folks country). On this trip we had launched in what is known as the Barge Canal just north of Crystal River, and ran that out to some islands that were formed when they were dredging that canal. Gorgeous area, you can see a tiny pic in my avatar

BTW, the phone was on the boat. There would have been no way to get to contact a tow. We would have had to wave down a passing boat, and we have very little traffic up here compared to the area you're talking about! That experience behind us, we are MUCH more aware of the tide any more!