Hard to get to plane when towing a tube

[sammith]

Hi all,

Brand new member to the forum (and to boating in general), so I apologize if some of these questions might seem a bit basic.

So here's the background. Bought a 1986 Bayliner Capri 1800 bowrider about 2 months ago with a 1979 Mercury V-150 2-stroke outboard motor. Great boat, loving it so far. The previous owner told me "it doesn't like to go slow", which I've started to notice is VERY true. It wasn't much of an issue until I bought a 3 person tube and went out tubing the other day. With just 2 adults on the tube, sometimes I would be at WOT and it just wouldn't go fast enough to plane, it would just struggle along at slow speed. Once it gets to plane though, no problem, full speed ahead!

I decided on the fly to do a rough WOT test (3 adults and 2 toddlers on the boat, plus I didn't trim up as much as I probably could have), and at WOT, I was going about 50mph at only about 3500 RPM. The book shows WOT should be 5000-5500 for this engine. I did some reading on props and from what I understand, I could probably go down 2-4 inches in pitch.

The prop on the motor is a modular 3 blade 14.5x19 (I think the brand is Piranha since it's the only brand I've seen that make these props, but I can't find any identification anywhere, so I'm not sure). The original engine that would have come on this boat (125hp) calls for a 13x17 prop, so I assume the larger diameter for this engine is normal, but it seems to me that the pitch is geared towards top speed, which isn't really what I'm aiming for, especially if I'm going to have fun towing people around.

While researching props, I found this forum that talked about how the height of the outboard could also affect acceleration/speed. I did a quick test with the information I found online, and I think my outboard is about 2 inches below the "normal" height (cavitation plate about 2 inches below the keel of the boat). What's more, looking at the mounting points, the motor is mounted on the 2nd highest hole (out of 5), with each hole 3/4 of an inch apart, I assume I could go down 2 holes (1.5 inch) to the 4th hole.

So I suppose it's time I actually ask a question (OK, many questions). Here goes:

1. What pitch should I try instead (I'm thinking around 15), and should I change the diameter? (I plan on getting a second set of blades for my prop either way, so I might as well get something different from what I have now).

2. Should I bother raising the motor? I would like to think it would be a good idea, but it seems like a lot of work (the 2 bottom mounting bolts are deep inside where the onboard fuel tank sits). I'm sure that just changing the prop will help, but if there's a chance that raising the motor will have a significant impact, then maybe it's worth the effort?

3. Does anyone know if there's another company that makes these "modular" props other than piranha? I would hate to order new blades and find out they don't fit the hub.

Thanks in advance! (and sorry for the novel)

 

[R055]

First thing you should do is confirm that the tachometer is accurate. 50mph sounds like exactly where the boat should be at 5k rpms. Also use a gps as a speedometer. Boat speedometer could also be inaccurate.

 

[89retta]

Start with raising the engine. With it sitting that low its like dragging an anchor. That will make a big difference in performance. Then get a good quality prop once you know what your rpms are after raising the motor.

 

[sammith]

First thing you should do is confirm that the tachometer is accurate. 50mph sounds like exactly where the boat should be at 5k rpms. Also use a gps as a speedometer. Boat speedometer could also be inaccurate.

I had considered the fact that the gauges might not be accurate, but I didn't double check. How would I go about double checking the rpms? I'm sure there are tools made for that, but if there's a way to do it without having to buy a tool, that would be ideal.

89retta: How high would you recommend raising the motor? Up to the keel? Or even higher?

 

[jimmbo]

Raising the engine is fine with boats that go faster than 50 mph and using higher performance props(more cupping and higher Rake), ie) Laser, Enertia, Vengeance, Tempest to name a few. I'm surprised you are doing as well as you are with the prop you have, but it will not do well at a higher transom height.
Raising your engine is not going to be easy, it does not have a transom bracket that allows for easy raising, and to make it even more difficult, during those years, merc used hollow lower bolts to pass the trim system fluid through the transom. They were/are easily damaged and the fact riggers apply sealer in the transom holes before installing the bolts often caused issues.
If you insist on raising the engine, get your self a suitable prop and you might be able to get 1" -1 1/2" tops. Get a water pressure gauge too, and watch for water starvation, it will happen at higher heights and trim settings

 

[Scott Danforth]

I had considered the fact that the gauges might not be accurate, but I didn't double check. How would I go about double checking the rpms? I'm sure there are tools made for that, but if there's a way to do it without having to buy a tool, that would be ideal.

you spend some money on a photo-tach

https://www.amazon.com/AGPtek-Profe...d=1499855626&sr=8-3&keywords=photo+tachometer

 

[89retta]

Jimmbo why I said to raise the motor is because he states that the cav plate is 2" below the keel. Should be even with the keel

 

[ahicks]

Raising the engine isn't going to help accelerate the boat near as much as lowering the pitch will.

Mounting height is more about top speed and dealing with cavitation/ventilation issues when accelerating hard out of the hole or maybe when turning hard. As our OP isn't too concerned about top end, and there is no apparent issue with the engine over revving getting the tube on top, engine height here is a secondary issue, if it's even an issue at all.

A VERY casual tachometer confirmation can often be done using the engines idle speed. If it's indicating as it should, showing an idle speed in the factory suggested range, it's very likely giving an accurate reading at higher rpms. When they're off, usually they're WAY off. An example might be a tach showing the engine idling at 500 rpm, or maybe 1500 rpm, instead of the more normal 900-1100 rpm. That's been my experience anyway.

3 guys on a tube is a big load for a boat like this (especially 3 big guys), and especially if it's also lugging a lot of gear in storage areas and maybe a lot of passengers. I would suggest if tubing will be a priority, lighten the boat as much as possible. You'll have much more fun.

17" pitch props are really popular on similar runabouts, but you could certainly drop down to a 15" assuming the numbers you show are accurate. Which to go with would probably best be determined by your priorities. If this is going to be a ski boat, go 15" If it's going to be used as a combination ski and cruise, the 17" might work better.

Last, as the 19" you have now is paid for, maybe look into having that prop repitched lower at a good prop repair facility (to be used for cruising), and buy a 15" for hard play at the end of a rope!

 

[sammith]

Thanks everyone for the replies. I'm starting to get an idea of where I'm going with this (starting with confirming my tach reading).

jimmbo: My merc doesn't have hollow bolts on the bottom, so I'm not worried about that. But raising the motor is still something that I might end up leaving up to a professional if I decide to go that route. And yes as 89retta said, right now my motor is at least 2 inches below the keel, so I would just be raising it to bring it to its "normal" setting.

Scott: Thanks for the photo tach idea. I feel kind of stupid now since I learned about these in college, but I haven't used one in years so I forgot they existed. I'll check at work, maybe we have one laying around.

ahicks: Thanks for your insight. I'd like to say my tach is reliable, but it doesn't even register below about 1200, so I'll double check it for sure. I don't think I'll ever have 3 adults on the tube (for one, the weight limit of the tube is 500lbs, and for seconds, I can just picture my boat taking a sharp corner and leaving the transom behind), so I don't think I'll go more than 2 adults and 1 child (400 or so lbs) to err on the safe side. But yes, I would like the boat to react a bit quicker when I'm tubing and if possible to just "enjoy" going a little slower. I figure I'll start with a prop and go from there (oh and my prop is plastic so AFAIK it can't be repitched, but it's modular, so I only have to buy new blades instead of a whole prop).

So thanks again everyone for the replies. I still need to figure out what pitch I'll get (still leaning towards 15 I think to make sure I get a noticeable difference from the 19), but I'll do some more research and talk to more people and make up my mind from there. As for the motor, I think I'll wait until I get a new prop and reevaluate from there.

 

[NYBo]

If you have the original tach,
it is set up for the Force outboard the boat came with. Since the Force has a 20-pole alternator and the Merc likely has 12, there's your tach error. If my math is correct, 3500 rpm is really about 5800.

There is no way you are doing 50 mph with a 19" prop if the motor is only turning 3500 rpm.

 

[sammith]

If you have the original tach,
it is set up for the Force outboard the boat came with. Since the Force has a 20-pole alternator and the Merc likely has 12, there's you tach error. If my math is correct, 3500 rpm is really about 5800.

There is no way you are doing 50 mph with a 19" prop if the motor is only turning 3500 rpm.

Hmm.. Interesting. I never considered that. I don't know anything about the original motor except that it was 125hp, but I see that you have a very similar boat to mine, so I'll take your word for it. That might also be the reason why the tech doesn't register at low speeds.

I'll start researching this some more tomorrow, but in the meantime if you have any information on doing a proper conversion so I could eventually rely on my tach, I would appreciate it. Thanks!

Edit: so I'm going to jump the gun and ask this question: let's assume I find out my tach reading is off and that my WOT is actually around 5800, I obviously won't be able to pitch down since even a 2 inch drop would raise my RPMs by 400 (if I read correctly), so would my only option at that point to try and improve acceleration be to raise the motor?

Edit 2: Just had a look at the original brochure for the boat and yes it's a Force motor that came stock.

 

[NYBo]

I'll start researching this some more tomorrow, but in the meantime if you have any information on doing a proper conversion so I could eventually rely on my tach, I would appreciate it. Thanks!

Your conversion factor would be 5/3 (20 pole divided by 12 pole), or 1.67.

There may be a selector switch on the back of the tach.

 

[jimmbo]

Jimmbo why I said to raise the motor is because he states that the cav plate is 2" below the keel. Should be even with the keel

I read that he "thought" the AV plate was below the bottom. Until verified with a straight edge... Trimmed all the way in/down, the AV plate will be lower than the hull
I find it hard to believe a boat of that vintage would have an 18" transom, unless it has a pronounced keel sticking out below the hull. Then it would need to be deeper

 

[sammith]

I read that he "thought" the AV plate was below the bottom. Until verified with a straight edge... Trimmed all the way in/down, the AV plate will be lower than the hull
I find it hard to believe a boat of that vintage would have an 18" transom, unless it has a pronounced keel sticking out below the hull. Then it would need to be deeper

Not sure about the size of the transom, but I did lay a yard stick on the cavitation plate with the motor trimmed all the way down and the stick was a couple of inches below the keel.

NYBo: I thought of that too, and I'll check to confirm, but after a quick Google search yesterday, it seems these tachs (US Marine Faria Gauges) usually have no settings or just a setting to select the amount of cylinders. To be confirmed once I check my actual tach.

I also read up about Tiny Tach, I might look into getting one of those if need be.

 

[jimmbo]

Place the straight edge along the bottom of the hull extending past the engine. Then trim the engine so it is parallel to the straight edge, actually a few degrees out from parallel as the boat does ride a few degrees bow high. How deep is the AV plate then?

 

[sammith]

No Title

Place the straight edge along the bottom of the hull extending past the engine. Then trim the engine so it is parallel to the straight edge, actually a few degrees out from parallel as the boat does ride a few degrees bow high. How deep is the AV plate then?

Here's a picture of how I did it last time. Hard to see from that angle, but the yard stick is nearly parallel to the keel. I'll try doing it again completely parallel, but considering how close it is, I'm guessing it won't change much. I've never done this test before, so if anyone sees anything else I might have done wrong, please let me know

 

[sammith]

Update: Levelled the outboard, rechecked and it's about the same with the yardstick sitting on the plate. What does change the test a bit is putting the yardstick on the bottom of the boat, since that seems to have a bit of a different (better maybe?) angle to it. Doing it that way, the yardstick sits about 1 inch above the plate. Factor in the thickness of the stick and it's maybe 1.25 inches or so.

I also checked the tach while I was out there. It has an adjustment on the back (small black arrow that you can turn), but everything I read online seems to indicate that it only adjusts number of cylinders, not poles. But I also saw on another forum here on iBoats something about a Tachometer Divider (or converter?) that might help. I'll have to do some research on that. Here's the link to the one the person on the other forum indicated:

http://www.bakerelectronix.com/products_tsd/

 

[jimmbo]

Well a picture is worth a thousand words. You are nowhere near the keel, you are measuring off a setback( a built in transom bracket). Also explains the odd 18 inch transom measurement. The gray arrow points to the keel(bottom of Hull) Put the straight edge along on the keel(you may need a longer one and some tape to secure it to the hull) and see where it is in relation to AV plate is(don't forget to trim engine out till AV plate is parallel to the straight edge"

 

[NYBo]

Update: Levelled the outboard, rechecked and it's about the same with the yardstick sitting on the plate. What does change the test a bit is putting the yardstick on the bottom of the boat, since that seems to have a bit of a different (better maybe?) angle to it. Doing it that way, the yardstick sits about 1 inch above the plate. Factor in the thickness of the stick and it's maybe 1.25 inches or so.

I also checked the tach while I was out there. It has an adjustment on the back (small black arrow that you can turn), but everything I read online seems to indicate that it only adjusts number of cylinders, not poles. But I also saw on another forum here on iBoats something about a Tachometer Divider (or converter?) that might help. I'll have to do some research on that. Here's the link to the one the person on the other forum indicated:

http://www.bakerelectronix.com/products_tsd/

Number of cylinders is only relevant when the tach signal is coming from the ignition coil. Yours is coming from the alternator. Set yours to 6 (half the number of alternator poles). What is it set to currently?

 

[sammith]

Well now, don't I feel stupid! Worst part is that I thought of asking that question before, but I figured "nah, there's no way to take the measurement off something that isn't flat". Guess I was wrong. Lol. That'll teach me... ok so I could do the test again, but judging by the fact that the area you've indicated is a lot lower (at least thick enough to fit a drain plug, so what, maybe 3+ inches), the motor must definitely be high enough, maybe even too high.

NYBo: I don't know what it's adjusted to. I took a picture, but the markings that should usually be there are all but gone. I'll probably have to pull it out and go all the way to one end and count the clicks. Or I can always find out how many cylinders a 125hp Force engine would have had back then and assume that's where it's set. Back to the books for now!