HPX-V 18 115 SHO Ride Quality

[17macae]

I've been through quite an adventure getting this boat working properly. I bought it used and it came with a 115 SHO, power pole, jack plate and 4 blade spitfire 21 prop. She was very unstable above 35 mph and I could only control her with a lot of tab. 

I removed the PP and JP and replaced the prop with a 3 blade 21 yamaha. So the engine is now 4 inches forward and the stern is about 90 lbs lighter. I mounted the engine in hole 2 from the top. The plan was that this would help the porpoising and control issues.

Anyway, the boat is manageable now. I can go full throttle with no or a small amount tab but she can still get out of control and require more tab. I'm use to a MA17 where I only used tab to deal with the conditions. I run very light with 2 batteries, 1 front and 1 center, and usually one person. I've got her up to 48 mph so far.

Does this sound normal for these boats? At what height are your motors set? 

[Tender]

I don't have a SHO but a F115. It is mounted on the lowest position and I run a 4 blade stern lift prop. That Yamaha 21p is probably Ok but if you find yourself prop shopping a 19p 4blade Merc Spitfire or 20p 4 blade Yamaha Talon would be on my shopping list. Not sure where your batteries are but I have 2 in the front and starting in the console. The boat is going to be more weight sensitive than your MA was.

[geeviam]

I agree with the weight sensitivity mentioned above.  If you bought the Yamaha Pro K-21 used, it may not be at factory specs, and that could be a problem.  Several more things you could try.  To start, you could install a wedge kit (Bob's Machine Shop) for more negative trim, to allow you to settle down the bow with less trim tab use.  It helps with hole-shot as well.  Your motor may need to be dropped down to the top hole, to get your prop blades into cleaner water with less air mixed in (less prop slip).  The correct motor height is when the underside of your motor cavitation plate, when trimmed level, rides on the surface (on top) of the water stream - not higher than that, unless you're running a custom XXX cupped prop.

Lastly, the prop...  You need a high-rake, cupped prop with good grip & less slip, to lift the bow and carry it without allowing the bow to fall into a porpoising rhythm.  It also needs to allow your motor to reach the top end of the recommended maximum RPM with a light load and just you driving (yours should be 6200 to 6300).  Your Yamaha Pro K-21 should be just right if it's true to factory spec.  You could send it to a reputable prop shop and have it reset to factory specs for about $150, and it would be better than new.  Before I'd buy another Merc Spitfire X7, I would try a Merc Trophy Plus 19 or 21 pitch (hard to find now).  I recently bought one and IMO, it spins better, grips better, and adds more stability than any Spitfire X7 I have tried - and I have tested the 17p, 19p and 21p Spitfire X7s on my skiffs.  Believe it or not, a 4-blade Merc Trophy Plus and a 3-blade Yamaha Pro will run the same max RPM with the same pitch on my skiff with a Suzuki 90.  Hope this helps.  Good luck!

[17macae]

Thanks for the feedback.

Went out yesterday in some nasty conditions and you definitely have to have your fingers on the tabs. Handled the whitecaps decent when trimmed and tabbed . Fished nice and skinny.

I moved the motor to the lowest position this morning. Are you'll telling me to give the 4 blade Spitfire another chance - have not run it since I reduced the stern weight?

[geeviam]

4 hours ago, 17macae said:

Thanks for the feedback.

Went out yesterday in some nasty conditions and you definitely have to have your fingers on the tabs. Handled the whitecaps decent when trimmed and tabbed . Fished nice and skinny.

I moved the motor to the lowest position this morning. Are you'll telling me to give the 4 blade Spitfire another chance - have not run it since I reduced the stern weight?

Yes, I would give that prop another try.  You've made some significant changes.  My good friend Dabear on this forum taught me that you should not make too many adjustments at one time, or else you will not know which adjustment did the trick.  A good lesson learned.

[17macae]

Ran the 4 blade spitfire and no luck.  Porpoising that had to be controlled by the tabs throughout the throttle range. Top speed 45 mph with tab.  Before I reduce the weight, the top speed was about 40 because of heavy tabs. It jumps on plane very nicely. 

I'm going back to the yamaha  62A-45970-10-00.

The boat draft is nice.  I was floating enough to fish where the reds backs were barely underwater.  Let the tide push me onto the flat as the water rose.

 

[geeviam]

1 hour ago, 17macae said:

I'm going back to the yamaha  62A-45970-10-00

That's a good plan.  The Spitfire X7 is too small in diameter for your rig (12.7" diameter).  If you want a 4-blade, you should try the Merc Trophy Plus 21 pitch or 19 pitch (13.8 " diameter).  Was the Yamaha Pro 21-K new or used when you bought it?  It should provide good bow-lift and reach 52 mph at 6000+ RPM on your rig.  If not, you should probably send it to a good prop shop and have it reset to factory specs.  I can recommend a shop if needed.  Do you have a torque/trim tab zinc anode with fin on your motor lower unit behind the prop?  If so make sure, when the motor is tilted all the way up and you're standing behind the boat looking at the underside of the cavitation plate, that the zinc anode trim tab fin is lined up between 12 o'clock and 1 o'clock (basically, straight back, maybe aft end favoring starboard one tooth).  Or cut/grind off the fin - flush, because you don't need it anyway.

One more question...  When your trim tabs are retracted all the way up, is every part of them (hinges and trim planes) above the bottom running surface of your hull?  The water stream should be running past them without any drag when they a fully retracted, meaning they should be flush, or slightly above flush, when not used.

[17macae]

I bought the prop on ebay out of seattle,  it looks brand new.  I can't see any sign of reconditioning. The only markings on the prop are 13 1/2 X 21 K.

The trim tabs and anodes are all as they should be.

I have not got it above 6000 rpm yet - it's been a little windy and scary approaching 50 mph in this little boat. I not use the noises and vibrations yet.

[geeviam]

20 hours ago, 17macae said:

Porpoising that had to be controlled by the tabs throughout the throttle range

Most, if not all stock flats skiffs porpoise at some point in their range of speed.  It's just the way it is.  They need trim tabs to create the hook and stabilize the ride.  Today's high performance bass boats are designed with a hook and reverse cupping in the chines and strakes on the last foot or so of the aft portion of the running surface of the hull.  They don't porpoise - even with the fuel tanks in the rear of the boat and a heavy big-block outboard hanging out there, on a 10"+ jack plate, on the transom.  They don't need stern-lifting props or trim tabs either.  They utilize bow-lifting props and run up on the pad as steady as you please.  Maybe someday, flats boat designers will try some of the same ideas, instead of the status quo.

[Tender]

Agree with Geeviam, some porpoising is to be expected at different trim levels and speeds. Mine will porpoise with a lower speed cruise (~28) if you trim it, if the motor is buried it will not. Sweet spot for me of cruise and trim seems to be in the 35-38mph range, seems to like a little more speed to keep the bow settled. Yesterday WOT 6000rpm 48mph 2 people, cooler, dog and trolling motor. No tab, RPM's started at around 5300 before trimming.

[Wanaflatsfish]

suggest you contact Marcus at PowerTech in Lousiana for recommendations on the prop as well....he's been my go to for 20 years.

 

dc

 

[Headhunter]

Try a cavitation plate from Bobs machine. It did wonders porpoise wise for me but I have a 150 and a jack plate. My buddy with a 115 added one as well and he doesn't use tabs at all anymore or porpoise. I also went with a powertech ofx 4 blade and a negative wedge. The boat run square now up to 5800rpm, no tabs, no porpoise.

[17macae]

I've been out quite a bit and have learned to just leave "one hit" on the tim tabs.  I can remove the tab at WOT, 5700 rpm and 50 mph on a hot day.  Going into turns it will get wild with porpoising and chine walking.  So, I have to back off and trim some then reset on straight runs. The bow is still light with the minimal trim so I guess it's better than having that trim designed into the hull. I have not run it with two people yet.  The fish I'm on are so skinny that I might not be able to get to them with the extra weight.

I just figured out how to replace readings on the 6yc gauge so I have my water speed displayed along with gps speed. She does well with the gas and I have not had to fill up since I calibrated the gauge. It's tough to launch with the Ameritrail trailer so I have been adding gulf wax to the bunks.

[Tender]

At 5700 you still have some rpm to go, it will definitely feel light and I only typically run over 45 in light chop or slicked out conditions and yes when entering a turn porpoising will happen unless I trim down or tab down at those speeds. Sounds like you're getting dialed in.

There has been a lot of talk about bunk slick since liquid rollers has been discontinued. Quite a few have said that Gulf Wax made their boats stick and because it is a wax it was impossible to get out of their carpet and they ended up replacing it. I have a can of Liquid Roller left and normally spray the front 1/2 to 1/3, the part that never even enters the water and my boat slips right off.

When I run out of Liquid Roller I will probably try silicone 1st unless liquid roller reformulates and comes out with a new product.