Setting up an outboard motor on a boat

[eng208]

I have a question about set up. On one of my small inflatable boats at work, I have a Nissan 9.9 mounted. It is on a small 10' inflatable that can be handled by two firefighters at a scene. It is for a rapid response for ponds we have in the area. I also have a Zodiac FC-470 I set up for rescue and dive operations when something bigger is needed.

The smaller boat will run about 15 knots, but is almost scary at that speed. VERY twitchy. I also found, when running at that speed, I get a large amount of water spray from the shaft of the engine. Enough that it will spray over the transom, regardless of trim angle. It is a short shaft, which is the right measurement for the transom. What causes this?

It does it on the Zodiac also. I have a 25 HP Mercury 4S long shaft on it. It needs the long shaft to avoid cavitation. It will run about 20 knots wide open with a light load and trimmed out.

It is pretty cool being able to just hook up to the rescue boat and go out on the lake when things are going well. Doing some training.

 

[mommicked]

Re: Setting up an outboard motor on a boat

I'm no expert, but I think a certain amount of spray would come w a shortshaft motor on a small inflatable. I would also think a wide, 12 or 14'+ johnboat would be more rugged/ less likely to be punctured, last forever, and would be usefull/easier for getting into and out of ponds w/o any type of ramp or launching area because 2 could drag it to the water if you had to. I believe it would be easier to haul someone aboard the inflatable, but how often would that happen on a pond?? what size of ponds are you needing it for, I think an emergency or rescue on a small pond is probably going to include a drowning because most folks could swim to shore if they end up in the water from a capsized or sinking boat there. But again, I'm no expert.

 

[Texasmark]

Re: Setting up an outboard motor on a boat

On the front of the lower unit just above the antivent plate is another small plate that juts out front of the lower unit. This plate is a spray deflector and deflects the water that comes up the front of the lower unit when it goes through the water. If your engine is low enough that this plate is under water you will get what you got.

If you are running a 20" on a 15" transom, expect this problem. Don't fully understand how many boats and how many engine combinations you are talking about.

Measuring the transom height is easy, just measure it; clamp bracket top to bottom of hull. If somewhere around 15 that's what it is. If around 20" that's what it is.

HTH,
Mark

 

[cyclops2]

Re: Setting up an outboard motor on a boat

Scary ???

Hell yes.

10' is WAY to small of a boat for a 9.9 hp engine. 2 men & rescue gear in that boat is a accident waiting to happen.

Excessive prop spray is USUALLY from a motor & prop too high up....Inflatibles can float very high up. Even when fully loaded compared to a V hull shaped boat.

 

[eng208]

Re: Setting up an outboard motor on a boat

I had a 14' Jon boat at previous dept. Did not work well for getting in and out of boat. The Zodiac is much easier to work off of and 100 times more stable. Good inflatables are actually pretty rugged.
I need to adjust the WOT on the small boat. I built the trailer set up. It is very easy to launch and has a tilt feature. Our ponds are pretty big around here. Four guys can lift the zodiac and move it from the trailer.
I think the spray is a function of the ride height of the boats. The small boat definitely uses a short shaft and the Zodiac transom height is 18".

 

[mommicked]

Re: Setting up an outboard motor on a boat

How wide was the previous 14' johnboat? I know quality inflatables are rugged and lightweight, but I believe sturdy ones w floors,etc probably weigh as much or more than a sparsely equiped, at least 48" bottomed jonboat. I'm sure the inflatable is better for getting in and out when out in deep water and w dive gear and less tippy when going over the side, but not 100 times better and you can buy or rig a step or ladder to make it easier to do from a jonboat. A jonboat can be pushed and or or dragged or carried to the water quickly in an emergency if you cannot access the shore easily and launch from the trailor, and no chance of popping an inflated boat used the same way, which would then be useless in any rescue or situation where time was critical. On small or large bodies of water I would think a small or large johnboat would be more usefull and dependable than a zodiac type boat. Rescue is one thing, recovery is another and not much need for speed or ease of getting in and out of the boat in the water for the later. Again i'm no expert but I have " moved " alot of jonboats to and from rivers, ponds, lakes and big creeks and 2 strong guys can get them up, down steep banks, through woods and thickets, mud, rocks, whatever w/o a trailor and quickly if you have too. Wide bottom jons are very stable at rest and at speed and can endure alot of abuse and still float.

 

[eng208]

Re: Setting up an outboard motor on a boat

I guess we all have opinions on what might work. I know I have been doing it 19 years and luck only gets you so far.
So far, I think the Zodiac is going to be far superior to the previous boat I had in the same role. It was a wide bottom btw.

I started this thread to get ideas about adjusting out the spray off the motor mainly and feel if I can do that, it may make the set up a little more efficient. Maybe I will play with the height a little. Adjusting the trim up helps, but leads to cavitation and slower speed.

 

[mommicked]

Re: Setting up an outboard motor on a boat

I opalagize for not giving you any info on fixing your problem, only an "opinion" on the type of boat your using for this service. I hope someone can help you improve what you have.

 

[Texasmark]

Re: Setting up an outboard motor on a boat

Tell you what. Since you have shown your competence in taking and posting pictures, take one more. Let the engine down to the normal run position. Get behind the boat low enough so that you can just about line the camera up with the bottom of the boat where you can almost see the bottom aft to forward. Shoot the shot. I am looking for the position of the AV and anti spray plates to the bottom of the boat.

Mark