Discussion in 'Technical Discussion' started by CSkipR, Sep 26, 2020.
Was the stabilizer on when running? Make sure the fins are centered.
Does it have underwater exhaust?
I assume they are as he just had them serviced when the boat was hauled. The problem with planing was there before and is still there. We actually tried it both ways with them on and off.
Yes it does.
14 knots for a 54’ boat puts you into a semi- planing state, you are just on or barely off the planing hump region. Very load sensitive and quite odd that a Pilothouse design needs full tab as their CG is usually more forward than aft.
The ‘shudder’ is a key tell, it is usually coming from an appendage or the underwater exhaust flow being sucked into the prop, which can happen in shallow inter coastal waters.
If it is squatting big time and you need full tab that shudder may be underwater exhaust/prop interference which is a problem on itself.
Need to check in the er if or check fins underwater to see if they are centered. If the trim tabs are working it is likely the stabilizer fins are not centering. Could be an issue from the recent service. If the fins are off center or not operating correctly with underwater exhuast one side will likely cavitate with exhuast at some point.
From everything you've checked water intrusion remains my guess, and this could account for it. Still putting a meter to the hull next time you haul would be a good idea.
Just to cover all bases, in looking back to your OP I see 2 things not mentioned: Struts and shafts. Were they checked for true? Also motor mounts and shaft logs.
Anything is possible but this problem was there before the boat was pulled and after the haulout. The reason it was hauled is because of the not planing issue. While it was out he decided to have bottom painted, stabilizers serviced, and props serviced.
If you're certain all of the running gear is good, then I'd start looking at the engines and possibly not making as much power. When it's shuddering go in the engine room and see if it's one engine shuddering or both......should be easy to spot.
Struts and shafts were not checked. The motor mounts also were not checked. The strange thing is this issue popped up from one week to the next week when he runs it to keep the bottom clean. Those are all things we will need to check.
Will do that.
Since prop work was recently done, relook at the paperwork to make sure their numbers are right. And make sure they are on the right shafts, port and stud.
But the inherent problem of needing full tab can be addressed in another way. The use of full tab to plane is a sign of imbalance, weights, centers of gravity and planing hull area are not well harmonized.
Check the travel of the trim tabs out of the water or at rest with a diver. Are they level (neutral) with the hull bottom at full up position or above the hull bottom (negative) or below the hull bottom (positive) at rest. Now check what is the full stroke of the tab , from full up to full down. A proper installation should only require half the stroke under normal conditions.
This hull sounds like it needs some hook added to the hull bottom to help her perform. When it is out of the water, get an 8 foot straight edge and check the last eight feet of the hull, port and stud in the fore and aft direction. There should be no voids or hollows as this will create suction. After you are sure that the sections are straight, a builders trick is to add some hook.
Looking at the transom bottom edge you would grind and prep the fiberglass 18” forward of the transom bottom edge. You would then build up a frp wedge that goes from approximately 1/2” to 3/4” tall at the transom bottom to 0” at a point 15” - 18” forward of the transom with solid glass. Fair it smoothly. This may require repositioning of your trim tabs to make them start at a neutral position.
I have used this process on a few Pilothouse models others designed with always positive results, the hook alleviates the need of full tab and helps you get on plane. It can also be done in reverse to add “rocker” for boats that plow or run too shallow trim angles at speed.
CSKIP- also try it with the trim tabs centered and see if that makes a difference.
Do you really think an owner is going to re-design the bottom of their yacht? Or a yard is going to do this to the bottom of a yacht and put their stamp on it? Also doing what you suggest may have very unfavorable results at hull speed. I'd recommend putting a 1" wedge between the trim tab rams and the tabs to give it an additional 1" of downward travel, this does A LOT in my experience on yachts that need it as tunnels don't generally allow larger tabs.
The offshores have a crappy hull design (if you're trying to push them over hull speed), and weight distribution, like most of the Taiwanese tubs, which is how they got nickname in the first place. They also have some ill natured handling quarks over hull speed, like running a . The Taiwanese mostly do hull design by trial and error. Or one mold makes several different lengths and the 1 length actually handles correctly, but the 2 other lengths don't because they don't move anything inside of the yacht (tanks, major components) to make up for the shift in trim.
They don't pay attention to where they put weight, or even on what side of the vessel they put weight. I've found anywhere from 500-2000 lbs of lead on most of them, and over 1000 lbs many times. The one with 2000 lbs of lead was in the cockpit where the factory took an existing design and just put a cockpit on it, had the geniuses switched the 200 gallon fuel tank under the master and made it an extra 200 gallons of water, and put a 250 gallon diesel tank in the cockpit, you'd be carrying around extra fuel, instead of useless lead and it still was bow heavy and handled horribly in certain conditions. They're at boat building where the U.S. and Europeans were in the early 1970's, as far as technology goes in their hull designs.
There are a few exceptions, boat those are mostly with a hull designer from elsewhere and steep oversight from the builder, Hunt is a perfect example of that.
On our ole Bert, I just had larger tabs made.
It did bring the bow down better in shallow water.
After near 17 years on board, think were moving to the hill next year.
I'll be making some performance measurements of Ole Bert with live-a-board weight soon and compare to sparse weight later next year.
It's going to be a shocker for sure.
Ask your bud where is the outdoor grill and coal stored.
Bet a cold one he has a loaded boat also.
It is not that big of a deal to do, the better yards know all about hook and rocker adjustments, the go fast guys do it all the time when tuning their hull bottoms. It’s all glass work on the last 18” of bottom, theirs more installation risk in installing stabilizers or bow thrusters.
It may seem a lot if you have no experience with it but the results can be game changing.
I have PLENTY of experience with it, but there is LOTS of very ill handling quarks that could be introduced by doing it (adding hook or rocker), and no yard I know will take that liability unless a naval architect designs and approves it and even then most won't. What happens if the boat broaches in an inlet, guess who gets sued. It's different when the manufacturer does it, and I've seen several good ones do it.
In the offshore situation, a cockpit extension would be the only way to gain speed and get it to run faster with more aft running surface........Adding hook or rocker won't do anything worthwhile as it comes down to the design of the stern section and not having enough lift due to the design.
I remember an alloy hull had a cockpit extension added and it bow steered. Another similar hull came out of the water like a scaled dawg and flew, then one could not run a straight line even with the best hand or AP.
Here again, a proper design tech/eng is required.
Other than adding tabs that can be withdrawn if it's to much, The novice (most red neck yards and owners) should not mess with a bottom design.
Lets get the mother-in-law out of the bilges and get back to some factory trim weights before any engineering changes are really discussed.
I just deleted some more comments. I'll ad them later if it gets difficult in here.
Mist yards have a relationship with a Naval Architect. Hook or rocker on the last 12” - 18” of the hull bottom on an older boat is not a large liability issue if you understand the technical issues behind the problem.
Once again you get off track attacking a solution that you have no idea about for whatever reasons behind your desire to post your nonsense. Experience apparently does not always mean you understand the technical solution used in the design and engineering of planing hull problems and their solutions. Keep to the throttles and clutches, you are out of your wheelhouse again.
Let me get this straight. The OP's friend has a boat that used to plane and 2 months ago wouldn't come up on plane, and we're now discussing extending hulls rather than what might have happened 2 months ago? What's next, airplanes and space ships? Reminds me of some conversations from the 60's that usually ended with 'What was I talking about? Pass the bong'.
I know exactly what I am talking about and know plenty about hook and rocker. A lot of smaller boat builders built boats with a hook in the hull, and it made the boat run flatter, but also reduced top speed on the faster boats and most of the time people would be grinding the hook out to go faster, not add it. But in a slow yacht you're not going to be doing that. Have you even run a 54' Offshore or ANY Offshore? Probably not, especially from your arm chair. That being said, the boat did get up at run 14 knots in the past, it's shy on rpm's, and most likely a performance issue. That being said hook or rocker is where the stern angles down a little bit more then the hull midships to midships aft, yes it does create stern lift and forces the bow down, BUT, it also forces the bow down (albeit to a lesser degree) at hull speed and in a following sea can create some downright scary bow steering if there's too much hook, and these offshores already bow steer in a following sea, especially coming in an inlet with an incoming tide, your doing full turns to keep going straight. Had you had any experience in an Offshore, you WOULD KNOW THAT. Longer/wider trim tabs don't change the hull design and are a better solution to creating lift in most cases. But, obviously, Bobby's backyard boat yard knows more than Offshore at building boats and designing hulls.
Have you even seen an offshore out of the water? Have you seen the crazy shaft angle they have? Have you run an offshore? Have you even BEEN on one?