1970 Mirrocraft w/ Merc 500 loss of power, prop seems to push air, lack of water?

[Roadblock]

I have a 1970 Mirrocraft Titan 15 with a 1970 Merc 500 50HP.

Sometimes I just loose power when going. This isn't an engine issue, engine runs great. Seems to be like, um, the best way to describe it would be to say I'm running out of water around the prop. The RPM's come up and its like I'm sucking or pushing air and than it hooks up again finally but sometimes I loose a lot of power when this happens.

I checked the prop, doesn't seem to be spun.

If the lake it REALLY calm it doesn't happen so much but if there is any chop sometimes I struggle to get going or pull a tuber.

Also if I try to turn while going full speed it pretty much shuts down my movement. The motor doesn't shut down but the boat almost looses all power and just slows right down.

The motor is already drilled for a SE Sports 300 hydrofoil although I do not have one. The previous owner had one but kept it for another boat.

I know a lot of people hate these things but could this help my described issues?

I did try moving the fuel tanks and my cooler up front, under the bow and it does help a little bit.

I know I had a few hundred pounds of wet foam under the deck last year. I started to pull it up last year but discovered that the foam if the structural support, the boat has no stringers. I did not want the boat to be down all season so I stopped and I planned to just pull the new deck up this year and pull ALL the foam, put the deck back in and then cut some holes to pump in foam and go have it re-foamed. Haven't gotten around to that yet though. I did pull the desk over the winter though and let it all dry out. I checked it this year and took some core samples down inside the foam and it seems to be mostly dry now. The only water that has gotten in the boat this year has been what dripped off tubers or fish so unless I've got a leak I don't know about, its should be lighter this year. Still plan to pull the foam and re-spray though.

Like I said it pulls fine on a calm day sucks when there is a little chop.

Thoughts? Questions? Replies?

 

[smokeonthewater]

You are trimmed to high OR motor mounted too high OR have prop issues.

 

[UncleWillie]

What you describe sounds like Ventilation of the prop.

The easiest to correct is a Trim issue.
Start with the engine adjusted so it is parallel with the Hull.
It should be pushing straight backwards.
If it is tilted so the prop is pointing UP when the boat is dead level, this could be the problem.

Another common cause is mis-mounting.
The large flat plate, directly above the propeller, is called the Anti-Ventilation (AV) Plate.
Its purpose it to prevent Air Ventilating the propeller due to the suction in front of the propeller.
This plate needs to be very slightly Below, or just AT, the water surface when on plane.
This will place it at just about even with the Keel at the very bottom of the boat when mounted correctly.
Some people get away with mounting the engine with the AV plate an inch or so Above the keel.
It is a fine line as to where Ventilation will occur. You may have crossed that line.
If you have a motor that does not have a shaft length that matches the Transom height, this could be the problem.

A picture of your motor installation, taken from the side, and showing the AV Plate and the Keel, would help a lot!

 

[ondarvr]

The hydrofoil he took off is probably what made it work correctly, the primary function of one to help stop ventilation. Lowering the motor on the transom will help, if you can't lower it try another foil, it could solve the problem.

 

[Roadblock]

There is a pin that adjusts what you call "trim" and it's set to the motor is exactly dead level with the boat. Was set this way when I bought.

I'm unable to lower the motor anymore and the lower unit of the motor is pretty short compared to some boats I've seen which is kinda nice because I can run it pretty shallow but the prop does sit below the bottom of the boat.

Also UncleWillie, you may be on to something with the shaft being too short. Mirrorcraft listed this boat has having a long shaft motor, I know the Merc serial number on this motor comes up as short shaft. I don't believe this to be the original motor.

Maybe that IS why it had a foil on it...

My local Cabela's does have the SE Sports 300 Hydrofoil in-stock for $59.00 dollars, I might just grab one and put it on to see what happens as it's already drilled for it. If it doesn't help, I can just return it.

I might have to track down a long shaft lower unit, from what I can tell, you can swap out the lowers on this motor.

 

[Texasmark]

There is a pin that adjusts what you call "trim" and it's set to the motor is exactly dead level with the boat. Was set this way when I bought.

I'm unable to lower the motor anymore and the lower unit of the motor is pretty short compared to some boats I've seen which is kinda nice because I can run it pretty shallow but the prop does sit below the bottom of the boat.

Also UncleWillie, you may be on to something with the shaft being too short. Mirrorcraft listed this boat has having a long shaft motor, I know the Merc serial number on this motor comes up as short shaft. I don't believe this to be the original motor.

Maybe that IS why it had a foil on it...

My local Cabela's does have the SE Sports 300 Hydrofoil in-stock for $59.00 dollars, I might just grab one and put it on to see what happens as it's already drilled for it. If it doesn't help, I can just return it.

I might have to track down a long shaft lower unit, from what I can tell, you can swap out the lowers on this motor.

Get a camera and squat at the side of the boat looking right down the transom from the side so that we can see the bottom of the boat from the side at the transom and the engine in it's current position, dead level with the boat. What I want to see is the position of the anti vent plate, the horizontal plate just above the prop, with respect to the bottom. If you have a 15" shaft on a 20" transom, and my Mirrorcraft of the era was 20 inch, then it is understandable that you are having serious problems and the picture will clearly show that.

Get the picture and we'll go from there.

 

[smokeonthewater]

The foil isn't gonna solve the problem... 15" motor on a 20" transom isn't gonna work.

The anti ventilation plate (where the foil would bolt on) should be even with or @1" a I've the bottom if the boat.... If it's several inches above you need a longer motor or like you mentioned, the extension kit.

 

[ondarvr]

The prior owner had a foil on there for some reason, and possibly it worked well enough for him to use that motor. Since short shaft motors aren't always 15", and long shaft transoms aren't always 20" even though you expect them to be. A slightly lower transom and longer shaft, plus a foil could combine for a working combination. This is only speculation with the limited amount of of info we curently have available, one look at it could possibly lead to the correct answer immediately.

 

[Roadblock]

I know this isn't the photo you asked for but I'm at work and can't take anything until at least later tonight. This was prior to my replacing the transom, deck, and other restoration projects. I think I took this photo the day I bought it.

ANYWAY, it gives you a rough idea as to how the motor sits til I can take something better.

That fin on the motor where the hydrofoil would be mounted, its level with the bottom of the boat the best I've ever noticed.

Side note when I rebuilt the transom, I did NOT raise it at all.

 

[ondarvr]

It looks close to being correct.

If you think you can return the foil I'd bolt it on and see what happens. Quick and easy test that may work well.

 

[Texasmark]

1. Looks like your height is fine.

2. Don't understand the need for a whale's tail with the helm forward.

3. What do you have in the stern right in front of the transom?

4. Besides you driving, what other load is in the boat and where is it located?

5. When you are running at WOT where does the spray out to the side start? Know what I'm talking about. The spray that is foremost which indicates where the boat and the water first meet. An indication as to how much boat is in the water.

6. When you take a turn at pretty good speed, does the bow dig in, act bow heavy, plowing? Look at your stern, does it seem to rise?

7. Get back at the engine and crotch down looking forward from behind the transom. Do you see any dip or wave in the hull or is it straight all the way up to where the V starts forming?

8. Got any idea as to where I'm headed with all this? You may have too much weight forward and it's causing the boat to plow which forces the engine up rather than down and that is why you are loosing grip with the prop. Where is all that wet foam you mentioned? Your comment about moving things forward is confusing. Seems it'd be the other way.

9. Put them back and add some weight at the transom. Get about 4ea 60# sacks of Sacrete concrete mix and put them right in front of the transom, as close as you can ge. With your engine mounted as it is. Go and run that.....report the results.

10. That boat apparently has stringers as your deck is mounted to something besides the ribs. My Mirrorcraft was a 14' open fisherman and there were no stringers. Any flooring had to be mounted to the ribs.

Let us know.

 

[Roadblock]

Going to copy your list and go down and reply to each one and than update it as I can.

1. Looks like your height is fine.

OK, thought so too.

2. Don't understand the need for a whale's tail with the helm forward.

Not sure what you mean?

3. What do you have in the stern right in front of the transom?

Battery and two 6 gallon tanks.

4. Besides you driving, what other load is in the boat and where is it located?

Hmmm no other real load unless I'm carrying people and they would all be roughly located in the middle when sitting down. I will take a picture of how the seats are, but it's two back to backs.

5. When you are running at WOT where does the spray out to the side start? Know what I'm talking about. The spray that is foremost which indicates where the boat and the water first meet. An indication as to how much boat is in the water.

Not sure what you mean exactly.

6. When you take a turn at pretty good speed, does the bow dig in, act bow heavy, plowing? Look at your stern, does it seem to rise?

Stern digs in, bow comes up like there is too much weight in the back.

7. Get back at the engine and crotch down looking forward from behind the transom. Do you see any dip or wave in the hull or is it straight all the way up to where the V starts forming?

I do see a couple dips/waves from the boat sitting on the EZ-Loader all these years. I suspect the foam has compressed where the rollers sit just a little. Hopeing if I re-foam, it will push them back out?

8. Got any idea as to where I'm headed with all this? You may have too much weight forward and it's causing the boat to plow which forces the engine up rather than down and that is why you are loosing grip with the prop. Where is all that wet foam you mentioned? Your comment about moving things forward is confusing. Seems it'd be the other way.

I would have guessed too much in the back because I have a hard time planing out. remove one of the fuel tanks and kick everyone out of the boat, seems smoother. Also if I stand when at WOT and stand as far forward towards the windshield as possible, it runs MUCH better and turns MUCH better.

9. Put them back and add some weight at the transom. Get about 4ea 60# sacks of Sacrete concrete mix and put them right in front of the transom, as close as you can ge. With your engine mounted as it is. Go and run that.....report the results.

I can tell you if I bring my Colman Cooler and put it near the stern its worse.

10. That boat apparently has stringers as your deck is mounted to something besides the ribs. My Mirrorcraft was a 14' open fisherman and there were no stringers. Any flooring had to be mounted to the ribs.

The deck sits on foam...There are these little ledges that run down both sides of the boat that the deck sits on and than it would appear the boat was pumped full of foam that expanded up to the deck.

When I got this far into it, I stopped because I was finding no structual support. I emailed the VP of Mirrocraft and he confirmed, no stringers, the foam is the support. Told me it was DuPont 4LBS 2 part foam. The deck literally sits ON the foam.

 

[Woodonglass]

Based on what I can see in the photo it looks a bit high to me. Check it using this...

 

[Roadblock]

OK I will double check it but it's the two don't exactly match, it's only off by an inch of so.

I thought this boat was supposed to come with a Johnson 65HP when they were new, that is what all the original flyers show so my guess is that the transom was built to match that motor if its off any.

 

[Willyclay]

Back in the days before the internet and this great forum, I had a short-shaft motor that I badly wanted to use on a long-shaft transom boat. I cut the transom down to fit the motor and never looked back. Good luck!

 

[Texasmark]

Going to copy your list and go down and reply to each one and than update it as I can.

1. Looks like your height is fine.

OK, thought so too.

2. Don't understand the need for a whale's tail with the helm forward.

Not sure what you mean?

Whales tales are usually used on boats with too much weight at the stern including the driver be it on a tiller or side, rearish mounted console.

3. What do you have in the stern right in front of the transom?

Battery and two 6 gallon tanks.

4. Besides you driving, what other load is in the boat and where is it located?

Hmmm no other real load unless I'm carrying people and they would all be roughly located in the middle when sitting down. I will take a picture of how the seats are, but it's two back to backs.

5. When you are running at WOT where does the spray out to the side start? Know what I'm talking about. The spray that is foremost which indicates where the boat and the water first meet. An indication as to how much boat is in the water.

Just look over the side. At the very bow if you look straight out (90 degrees) you see no spray. If you moved toward the stern, at some point you would start to see spray shooting out and to the rear. Where is that point?





Look below the last picture. I have something else to say.

Not sure what you mean exactly.

6. When you take a turn at pretty good speed, does the bow dig in, act bow heavy, plowing? Look at your stern, does it seem to rise?

Stern digs in, bow comes up like there is too much weight in the back.

7. Get back at the engine and crotch down looking forward from behind the transom. Do you see any dip or wave in the hull or is it straight all the way up to where the V starts forming?

I do see a couple dips/waves from the boat sitting on the EZ-Loader all these years. I suspect the foam has compressed where the rollers sit just a little. Hopeing if I re-foam, it will push them back out?

8. Got any idea as to where I'm headed with all this? You may have too much weight forward and it's causing the boat to plow which forces the engine up rather than down and that is why you are loosing grip with the prop. Where is all that wet foam you mentioned? Your comment about moving things forward is confusing. Seems it'd be the other way.

I would have guessed too much in the back because I have a hard time planing out. remove one of the fuel tanks and kick everyone out of the boat, seems smoother. Also if I stand when at WOT and stand as far forward towards the windshield as possible, it runs MUCH better and turns MUCH better.

9. Put them back and add some weight at the transom. Get about 4ea 60# sacks of Sacrete concrete mix and put them right in front of the transom, as close as you can ge. With your engine mounted as it is. Go and run that.....report the results.

I can tell you if I bring my Colman Cooler and put it near the stern its worse.

10. That boat apparently has stringers as your deck is mounted to something besides the ribs. My Mirrorcraft was a 14' open fisherman and there were no stringers. Any flooring had to be mounted to the ribs.

The deck sits on foam...There are these little ledges that run down both sides of the boat that the deck sits on and than it would appear the boat was pumped full of foam that expanded up to the deck.

When I got this far into it, I stopped because I was finding no structual support. I emailed the VP of Mirrocraft and he confirmed, no stringers, the foam is the support. Told me it was DuPont 4LBS 2 part foam. The deck literally sits ON the foam.

I just stopped what I was doing. You have two things working opposite one another. When YOU do what usually counters a heavy stern, you have stern heavy responses. When you do things that usually counter the bow up too high, you have a high bow attitude. You know what I think? Your hull is flexing!

That boat is a 20 inch transom. The picture that Roadblock provided had it stated in the fine print in the picture.

Measure your engine from the hook in the clamp bracket......that fits over the transom, touching the top of it, down vertically to a point even with your antivent. plate. If you measure 19-21" you have a 20 inch midsection on your engine. Look at the engine in the picture. Notice the mid section is nice and long. If it were a short shaft it would look like somebody squished it. 15" legs on engines of that size look what they are....short.

I think once you verify that your engine height suits your transom, fix the hull and get the existing bows out of it and get the foam in it as the mfgr. told you.............THEN go after what's left of a problem. What-cha think?

Edit: I just looked at your first picture of the boat sitting on the trailer and that engine has a 20" mid section and is matched to that transom. Go and fix your deck (floor) and get out and test it.....look at the bottom before you go and see if the hooks are out of it.

Good Luck,
Mark

 

[Roadblock]

If you look at the picture above, the problem is the splash pan at the back is in the way. I would have to remove and then find some way to get the water to go out and not down onto the floor!

 

[Willyclay]

You know what I think? Your hull is flexing! Good Luck, Mark

Just in case you don't think what Mark said is possible, I can verify it happened to my 1966 Lone Star (Chrysler) Medallion II that had the same type of structural foam and no ribs or stringers.