Planing Questions

[jsalz]

First of all, Hello! I have been lurking and reading for a couple of weeks now and finally decided to register. Very helpful site so far.
My brother and I are very green when it comes to boats so bear with me. We just wanted to get on the water so we bought an old inexpensive 17.5 ft tri hull with a 74 50HP Johnson 2 stroke.
The motor starts instantly and seems to run quite well except for the top speed. I realize it is a heavy boat and probably an udersized motor but at best we are getting 10-12MPH. The motor came with a hydrofoil but does not have power trim and tilt. It seems like we are pushing alot of water. I was thinking of tilting the motor outwards from the transom using the tilt pin to see if that would help with speeding it up. The pin is in the bottom most position now so I was thinking of moving it up 2 more spots out of the 5. My question is, by tilting the motor, will we actually be fighting the hydrofoil to get the boat on plane? Should we just accept that this is as fast as the boat is going to go?
Any help would be appreciated.

 

[QC]

Re: Planing Questions

My guess is the prior owner put the hydrofoil on in an attempt to get this boat to plane better. However, I would take it off to start my experimenting. My next thought is that the boat is over weight for a 50 horse, but with some info we could help you determine that. Is there an engraved plate that notes the max horsepower for that boat affixed somewhere near the helm? That'll give us an idea what she was designed for. Also we could use the prop info. Diameter, pitch, number of blades, material.

Also, trimming out (pin placed further aft and up) is usually bad for trying to get a marginally powered boat to plane, but the foil could be fighting it. Assuming we have enough horsepower (might not be) I would:
a) Make sure the motor is running right
b) remove the foil (she may need it, but let's get some info without)
c) get everything out of the boat except safety items
d) start with very little fuel
e) leave the pin in the lowest position
f) use a low weight test pilot, alone
g) see what you get

Welcome aboard iboats!!!

 

[Captain Caveman]

Re: Planing Questions

I have five tilt holes and generally keep it on the second or third hole out, assuming me and fishing gear along with bow-mounted trolling motor. In general the wake comes off the side of the boat between half and 2/3rds of the way back when I'm on plane. If I have other people in the boat and/or gear I shift the tilt pin back to either the 4th or 5th hole depending on weight.

I'm running a 35hp motor on a 16.5 aluminum boat - not tri hull.

 

[haskindm]

Re: Planing Questions

Does the boat have a tachometer? If so, is the motor reaching its full RPM at WOT (wide open throttle)? If not, you may have the wrong prop. If the boat is not equiped with a tach, perhaps you could hook one up temporarily for testing.

 

[jsalz]

Re: Planing Questions

I looked for a plate but was unable to find one anywhere. I do know that the boat is a 1974 17.5ft Grew (built here in Ontario, Canada) but I was unable to find any info about the the weight or ratings from searching the web.
The prop is an aluminum 3 blade stamped as 13.75x13 is the first number the diameter?
Am I correct that when the foil was installed, they needed to drill into the av plate? Might sound stupid, but do I need to plug those holes if I remove the foil?

 

[jsalz]

Re: Planing Questions

Does the boat have a tachometer? If so, is the motor reaching its full RPM at WOT (wide open throttle)? If not, you may have the wrong prop. If the boat is not equiped with a tach, perhaps you could hook one up temporarily for testing.

Actually, that was going to be my next question. The box that the shifter an key is mounted to (remote control box?) has a rubber cover on some sort of plug. I was asuming that it was for a tach. Problem is I don't know what type of tach to buy. I've seen some at our local marine shop for around 200 dollars. I was hoping to find something cheap and dirty for around 50 bucks.

 

[Silvertip]

Re: Planing Questions

It's best to plug the holes but for test purposes it shouldn't have any significant affect. A piece of tape on the bottom and filling the holes with epoxy is all that's needed in the future.

 

[Silvertip]

Re: Planing Questions

It's best to plug the holes but for test purposes it shouldn't have any significant affect. A piece of tape on the bottom and filling the holes with epoxy is all that's needed in the future. You might make sure the throttle is opening fully. If the boat is on a trailer, I would stand well back from one side or the other and see if the engine is actually at a 90 degree angle to the boat itself. If the engine is tucked in (trimmed in too far) it pushes the bow down and hence the plowing. 90 degrees or even slightly tilted out slightly is the way to get on plane.

 

[jsalz]

Re: Planing Questions

Thanks for the tips guys. I'll let you know what happens.

 

[haskindm]

Re: Planing Questions

When I bought my first boat the mechanic went with us on the "shakedown" and hooked his shop tach up temporarily to make sure that the boat was propped properly. Perhaps you could borrow such a setup. Does the engine sound like it is revving up (5000 - 6000 RPM usually) or is it laboring and bogging down? Your ear should be able to give you a good indication. A properly propped and trimmed outboard should be screaming at WOT.

 

[QC]

Re: Planing Questions

The reason I am not concerned with RPM at this point is the 12 MPH. There is no way she reaches WOT RPM at that speed. If we can get her going better then we could talk RPM. I also agree with Silvertip on the angle, but wanted to see what would happen at the 1st hole, then the 2nd etc. . . . Don't want to change too many things at once, need some baseline info and one change at a time.

 

[Steve Mahler]

Re: Planing Questions

you really need to weigh the boat to make sure it isnt full of waterlogged foam, very common on old open bow boats. Often recycling centers or truck repair shops have drive-on scales that are cheap...start there; there is a possiblility the motor is not performing completely, easiest thing there would be a mechanic doing a full check up - compression, spark on both, carb clean, proper size prop for setup, etc. I will say that these motors can run on one cylinder very smoothly, but give no power - it could be that...

 

[jsalz]

Good News-Bad News Update

Good News-Bad News Update

Well, I decided to try one or two easy things at a time.
First,I replaced the plugs. I made sure they were gapped to 0.030.
Second, I tilted the motor up two spots so that it was in the middle position of the 5.
Third, we hit the water to try it out. Let it warm up for about 5 minutes.
Initially, the boat motor ran great. The bow came out of the water and we got on plane failrly quickly. We actually hit 26mph gps. - Woo Hoo - thats almost 3 times faster than we had got this boat to go so far.
Then, after about 5 minutes at WOT she started to slow, the RPM's dived (No tach yet but pretty obvious by ear). and we slowed to about 9 mph again. We thought it might be water in the fuel so we putted around the bay for a little while and then to our surprise she took off again, all the way up to 25 mph gps. Ofcourse, the RPM's died about 5 minutes later again and again we slowed to 9-10 mph.
I thought I would check the plug wires to be sure we had put the boot on all the way. With the motor running in neutral, I popped the case and as I was about to push down on the rubber boots for the plug, I brushed something with my hand and got zapped - nice!
Anyway, killed the motor, checked plug wires and they were OK.
Boat ran at low RPM's for about another half hour, so we decided to head in.
I am guessing that it could be one of these things:
1. Water in the fuel?
2. Fuel pump not operating properly
3. Fuel restriction in one of the lines - checked fuel pump filter and hoses from tank and bulb and they all seem ok and fairly new.
4. Electrical problem? Seems weird that it ran better than ever when I changed the plugs and then just lost power again?

Any ideas, or should I take her to the shop. I would say I'm pretty mechanically inclined. I have worked on cars ocasionally with success but I don't want to bite off more than I can chew.

By the way, I have a service manual for the 1974 Johnson 50hp.

Thanks for your help

 

[Nandy]

Re: Good News-Bad News Update

Re: Good News-Bad News Update

What you are mentioning is very indicative of fuel starvation. When the motor start slowing down have someone pump the fuel line bulb, if it catches speed you know for sure you have gas starvation. Then you can look at your fuel line connections. That is generally where you will find that the air is filtering in. Also make sure you have your vent open in the tank. It does not hurt to check the entire fuel system, all your lines, filters and hoses for a loose connection or dripping gas.

 

[donnymac]

Re: Good News-Bad News Update

Re: Good News-Bad News Update

Also make sure you have your vent open in the tank. That's the first thing I thought of.

 

[jsalz]

Re: Good News-Bad News Update

Re: Good News-Bad News Update

Nandy, we tried pumping the fuel line bulb but it did not change anything. I did find one hose that ties the two carbs together was very easy to remove. Whoever installed used nylon zip ties to tighten them down. Is this normally used to hold the hose onto the fitting or should I use the metal hose clamp type. For now, I really tightened the nylon tie and it seems to be holding.

Donnymac, I triple checked the vent. We did that the first night we had the boat and realized about 5 minutes later.

 

[jsalz]

Re: Good News-Bad News Update

Re: Good News-Bad News Update

Anyone care to comment on using nylon ties for fuel line connections. Are they OK or should I replace?

 

[Nandy]

Re: Good News-Bad News Update

Re: Good News-Bad News Update

Sorry for the delay responding. Being tied up in my own project! Ties are fine. I use the in my Evinrude between the carbs. I like the clamp type better. Might change them some day. Maybe your pump is not pumping right. That really sounds like fuel starvation. I think you could get some fuel hoses and install a temporary inline clear fuel filter at the pump out port. Try to tie wrap the filter to something so you are sure is not banging around. When the motor stops check if it has gas on it. If no gas then keep checking going towards the tank, if it has gas then check towards the carbs.

 

[Kiggsia]

Re: Planing Questions

Jsalz, if you see this post since it has been over a year since you wrote about your speed problem, if you still have that boat and motor, how are things with it these days?