Tunnel Hull cavitation questions

Kingtom · Oct 29, 2011

I'm new to power boats and we pick up a 17' 2002 McKee Craft Backwater with Yamaha 90 2 stroke last year.
The boat has what's called a tunnel hull and the boat motor seems to run free or what I call cavitate when turning or when engine is not trimmed enough. This is really bothersome and performance is poor in my opinion. Any explanations out there and recommendations ?

Thanks

Texasmark · Oct 29, 2011

Re: Tunnel Hull cavitation questions

My 2C: Tunnel hulls were designed to get you up (on plane) and out of shoal areas where water may only be 1 foot or less. That means that the prop is deliberately sitting high in the water and the sides of the tunnel cause disturbances in front of the prop especially if not going straight ahead.

Additionally, they have jack plates whereby you jack up the engine to get through the shallow spots and when you are clear of them you drop the engine down, getting the whole prop in the water, to perform in the wind blown waters that you have to go through getting to and from the "flats".

A cupped 3 blade prop, or a cupped 4 bladed prop would be your best bet in my opinion.

I wouldn't expect a flats boat (tunnel-hull) to perform like an ordinary hull because it's not.

HTH,

Mark

steelespike · Oct 29, 2011

Re: Tunnel Hull cavitation questions

Its called ventilation. Most boats require you to trim down when turning.
With the tunnel prop selection and motor setup are critical.
Please read "read this first"at the top of the page get us as much info as possible.
Your present prop size should be near the base of the blades or under the prop nut.

nwcove · Oct 29, 2011

Re: Tunnel Hull cavitation questions

just curious, but are you running a surfacing prop set up.ie high perfomance?

Texasmark · Oct 29, 2011

Re: Tunnel Hull cavitation questions

nwcove said:
just curious, but are you running a surfacing prop set up.ie high perfomance?

A Cleaver maybe....straight trailing edge?

1Crazymechanic · Oct 29, 2011

Re: Tunnel Hull cavitation questions

4 blade prop will definitely help in this situation

mommicked · Oct 29, 2011

Re: Tunnel Hull cavitation questions

I've seen a few of these Mckee tunnel boats and I like them. They look like great fishing machines w a V hull for big water, and the rounded tunnel for shallow running.

Texasmark · Oct 30, 2011

Re: Tunnel Hull cavitation questions

mommicked said:
I've seen a few of these Mckee tunnel boats and I like them. They look like great fishing machines w a V hull for big water, and the rounded tunnel for shallow running.

I haven't seen the boat but from what you said they covered both bases as you have two different situations that you are trying to cover with one boat and a lot of the time you have to settle for one or the other. I may google them up and have a look.

Mark

Kingtom · Jan 13, 2012

Tunnel Hull ventilation questions

Tunnel Hull ventilation questions

Although it's been some time since the post I appreciate the replies.
Per the above recommendation here are the propeller specs. found on the propeller on the Yamaha 90 right now: 13 1/2 x 15 -K.
A little more explanation: even while running about 3500 rpm on plane while making a mild turn the engine seems to ventilate, until backed off of plane.
I'm thinking a propeller change may help a little although the more I read the tunnel hull really causes the ventilation so I may be stuck with the shallow running and bad performance.
Any comments on propeller selection would be appreciated.
Again the boats a McKee Craft 172, about 1200 lbs. with 300 lbs yamaha, with tunnel hull per the pictures.
The boat is not equipped with a jack to raise and lower the engine.

Texasmark · Jan 13, 2012

Re: Tunnel Hull ventilation questions

Re: Tunnel Hull ventilation questions

Kingtom said:
Although it's been some time since the post I appreciate the replies.
Per the above recommendation here are the propeller specs. found on the propeller on the Yamaha 90 right now: 13 1/2 x 15 -K.
A little more explanation: even while running about 3500 rpm on plane while making a mild turn the engine seems to ventilate, until backed off of plane.
I'm thinking a propeller change may help a little although the more I read the tunnel hull really causes the ventilation so I may be stuck with the shallow running and bad performance.
Any comments on propeller selection would be appreciated.
Again the boats a McKee Craft 172, about 1200 lbs. with 300 lbs yamaha, with tunnel hull per the pictures.
The boat is not equipped with a jack to raise and lower the engine.

That looks like a stock alum yammie prop. You can expect problems with that on this hull. Get yourself a cupped SS prop in 3-4 blades. If you have ever run a conventional standard performance alum and a ss cupped prop on the same hull/engine combo you would know what I am talking about. I have and will run nothing less.

Mark

steelespike · Jan 13, 2012

Re: Tunnel Hull cavitation questions

I don't think a ss prop is a good idea if you run in the shallows and are likely to make hard contacts.
A 4 blade prop will help get up quickly and should grip better.
Its hard to tell but it does look like the motor may be a little high, especially if you consider the turbulance
that could be created by the tunnel. Solas amita has a good 4 blade. If there is a choice of diameter go for the bigger one.
Usually suggested to drop an inch pitch when going to a 4 blade but that would depend on your present wot rpm and speed.

mommicked · Jan 13, 2012

Re: Tunnel Hull cavitation questions

I've seen the Mckee I spoke of again and it has a black 4 blade prop on its outboard. The tunnel on this CC looked larger than the one in the pic to me, but it could be the perspective of the pics. I personally had high speed turning and rough water instability or cavitation/ventilation/blowout etc. and other at speed performance issues w my 1860, 90hp, slight semi V jonboat and an alum, cupped, 4blade Solas prop improved my overall perf. a great deal w better grip or bite than my factory Yamaha 3 bld alum prop. It now turns like a sportscar w fat tires! getting and staying on plane at lower speeds, and acceleration? and stoping speed is unreal. Panic stops are brisk and sometimes needed, when running shallow water

smokeonthewater · Jan 13, 2012

Re: Tunnel Hull cavitation questions

engine looks too high to me.... if it can't be lowered you might consider a setback plate instead

ondarvr · Jan 13, 2012

Re: Tunnel Hull cavitation questions

I have the same type of hull and it can be a little tricky to set up the first time. I couldn't tell by the pic if you could lower the motor, if you can it will help, but you may lose a little speed. The other option is to try that evil tool called a foil or fin, this is the correct application for one, it will (may) help to reduce the ventilation, it helped a great deal on mine. A different prop will also help, and is a better option than a foil, but will cost a great deal more.

mommicked · Jan 14, 2012

Re: Tunnel Hull cavitation questions

Solas alum. props here at Iboats store, less than $150 w shipping

Scott Danforth · Jan 14, 2012

Re: Tunnel Hull cavitation questions

I would also invest in a jack plate

Bob_VT · Jan 14, 2012

Re: Tunnel Hull cavitation questions

BEFORE you invest in anything.......... that is too high and drop the motor down about 1" You might be surprised.

Based on your photos it is a high mount looking at the motor bracket vs the top of the transom.

Texasmark · Jan 15, 2012

Re: Tunnel Hull cavitation questions

steelespike said:
I don't think a ss prop is a good idea if you run in the shallows and are likely to make hard contacts.
A 4 blade prop will help get up quickly and should grip better.
Its hard to tell but it does look like the motor may be a little high, especially if you consider the turbulance
that could be created by the tunnel. Solas amita has a good 4 blade. If there is a choice of diameter go for the bigger one.
Usually suggested to drop an inch pitch when going to a 4 blade but that would depend on your present wot rpm and speed.

Runs fine sir. You are talking about sand, small shells and a few rocks. May pick up a ding or two periodically, and it can rub the mirror finish on the blades, but that't about it. No biggie, I assure you...ran the Texas and Mississippi gulf coasts most of my younger life. You could step out of the boat and wade and your belt was above the gunwale....it was that shallow.....butttttt that is where the Reds (Spotted Drum) were, and that is where the tackle "bustin" action was. Had to crouch down and have the front of your torso parallel to the water to sneak up on these suckers. They would have bait fish/shrimp hearded up against the beach,way laying them in a feeding frenzy......just gotta watch the sting (manta) rays when wading....barbs are poisonous and make a bad/painful wound, hard to heal.

Mark

Capslayer · Jan 16, 2012

Re: Tunnel Hull cavitation questions

mommicked said:
I've seen the Mckee I spoke of again and it has a black 4 blade prop on its outboard. The tunnel on this CC looked larger than the one in the pic to me, but it could be the perspective of the pics. I personally had high speed turning and rough water instability or cavitation/ventilation/blowout etc. and other at speed performance issues w my 1860, 90hp, slight semi V jonboat and an alum, cupped, 4blade Solas prop improved my overall perf. a great deal w better grip or bite than my factory Yamaha 3 bld alum prop. It now turns like a sportscar w fat tires! getting and staying on plane at lower speeds, and acceleration? and stoping speed is unreal. Panic stops are brisk and sometimes needed, when running shallow water

I just bought this boat, and I am ordering a spare prop. It sounds like you have the performance thing all sorted out. My boat is an 1860 CC tunnel with a 90 4 stroke. I have the flotation pods on the back. Exactly what dimensions is this 4 blade cupped prop you speak of? This is an interesting topic, because there are many different manufacturers of boats with tunnel hulls. If we share what we know it allows new buyers to know what to do with their new boat.

JB · Jan 16, 2012

Re: Tunnel Hull cavitation questions

Capslayer, you really need to open a thread of your own rather than divert Kingtom's thread to your question. We call that hijacking, and it is poor internet manners.

Thanks for your cooperation.

Tunnel Hull cavitation questions

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