Stability rules of thumb?

Greetings, I am a new member here and would like to develop a feel for the magnitude of the stability changes that I would make by reducing the weight of my engine rebuild by replacing some or all of my cast iron components with aluminum and stainless components. I am currently in the process of rebuilding the motor and it is removed from the boat unfortunately so I cannot not make any estimate of the initial stability by looking at the period and damping of lateral oscillations when disturbed.

The boat in question is a 1989 single engine out-drive deep-vee Formula PC26 performance cruiser with a dry weight of about 8000 lbs, a wet weight of around 8800 lbs, and a maximum fully loaded weight in the area of 10,000 lbs. The weight change in engine components on the big block Chevy that I am contemplating is in the range of -200 to -330 lbs and the CG of the engine is very low at about 2' above the keel, maybe a little less. The vertical CG of the vessel, GM, GZ, or any aspect of the righting moment curve(s) including where it goes negative is unknown and the manufacturer apparently has no data on this. So the changes that I am considering are in the range of only -2% to -3.3% of the maximum vessel weight but of course it is located in the worst possible place for it's effect on lateral stability.

Does anybody have any feel for the typical stability of a high freeboard vessel like the formula PC26 and would a change like I am contemplating warrant a complete (and I am assuming expensive) stability analysis by a professional naval architect?

Thank you for any guidance or references that anybody can give me on this project!

 

Welcome to the Forum Alan.

For general reference, here is a link to a sister ship of the same age for sale - she also has a single engine (Mercruiser).
1989 Formula F-26 PC, Long Beach California - boats.com https://www.boats.com/power-boats/1989-formula-f-26-pc-8264179/

Re making your existing engine lighter by say 300 lbs , realistically I don't think it will have a significant effect on your stability data.
OK, the KG of the vessel might increase very slightly, reducing your GM slightly.
However I think you will probably have a lot more effect from carrying crew - if say a typical crew member is 200 lbs, and you have 3 of them standing in the cockpit, with a combined KG of perhaps a metre above the cockpit sole, then that might well reduce your GM by much more than the reduction in weight of the engine low down.

What is your stability like presently - have you ever had any cause for concern, even in a sort of worst case scenario, like having to turn around in fairly steep waves?

 

I'd agree with bsailor here; don't expect any dramatic changes. To get an idea about the present static roll stability you can perform a simple test. With the boat at quayside, start rocking her to find the natural rolling frequency. Measure the time for, say ten periods. Then compare the time (in seconds) for a full period with the waterline beam (in meters).

A rule of thumb then says that the boat is ok if the period is shorter than the wl beam.

 

Thank you for the response and welcome. Sorry for the Quote but I can't yet figure out how to reply without the unnecessary quote?

No real cause for any gross stability concerns to date, but to tell you the truth I hadn't been too cognizant of the changes in stability it in the past.

Yes agreed that a number of passengers on deck might effect the lateral stability more than my envisioned engine changes, but that would depend on the actual vessel CGz location of which I really have a hard time gauging. Of course it's the sum of all these weight additions (or subtractions) multiplied through their offset that results in the ultimate stability. I forgot to mention initially that the primary reason that I wanted to lighten the load was that over time I have noticed how nice the boat seemed to handle when I burned off say 1/2 a tank of fuel (270 lbs) or more and how I could get on a plane easier and plane at a lower speed - this is important depending on sea state and particularly as I get older and am not so interested in speed, but a pleasant and efficient ride. The fuel tank is located just forward of the engine at about the same vertical CG and I attributed this nice change in performance to a forward shift in CG as fuel is burned. I had never even thought of the vertical shift in CG I'm embarrassed to say, but possibly part of what I am feeling might just be a reduction of the righting moment also and the dynamics are becoming less snappy (stiff). It's usually only my wife and I, but what happens with the stability if I make the engine changes, I've got maybe 3-4 big guys on the boat, I'm nearly home with say 1/3 tank of fuel and the sea state is not favorable - you get the idea.

Not sure if this event is well known to the group or not but over the last few days as I researched lateral stability I came across a Youtube video of the capsizing of an 80' Norther Marine Trawler on its initial launch. If anybody hasn't seen it look it up as it's quite stunning. The bottom line (as determined by the US NTSB) is that two ballast errors were made by the manufacturer, one on the longitudinal center-line and one offset from the center-line. The fascinating NTSB analysis is in the link below, but in my opinion had the heeling ballast error not been present at launch (and possibly a shipyard error or two) causing the immediate capsizing, the other ballast error would have gone unnoticed and the resulting stability margin would have been much lower than designed and a capsize could have easily occurred hundreds or even thousands of miles from help. The shocking curve in this report is the "16 LT Ballast No Heel Moment" as that it could have gone unnoticed it seems.

https://www.ntsb.gov/investigations/AccidentReports/Reports/MAB1514.pdf

 

Thank you for your reply - yes I have recently come across that rule of thumb and I only wish that I had done that test prior to removing my engine as it would have given me at least a feel for my initial stability. You can bet that regardless of what I end up doing I will conduct that test upon the first re-launch!

 

Re achieving your cruising speed, and getting up on the plane easier - boats are a bit like people in that it is fairly easy to add weight without realising it - it sort of sneaks on board. And it can add up.
Something else that will also have an effect is how clean the boat's bottom is, and the outdrive leg and propeller. Just a coating of slime on the bottom with have an effect, and if you have weed and shells attached, that will have a much greater effect.

Re the circumstances described above, I think that the effect of a weight saving of 300 lbs low down on your engine will be very minor compared to the effect of the 4 big guys (whose CG's are relatively high up) - and with the tank only 1/3 full you will have what are called 'free surface effects' which will cause a slight reduction in stability - an analogy could be if you compare a yogurt pot filled to the brim with water, and then you then pour the same amount of water into a frying pan, and try to carry it - what happens? Which one is easier to carry / has the better 'stability'?
And sea states can easily aggravate / compound any existing problems.

Here is a link to the Northern Marine video that you mentioned -


I hope that you will be pleasantly surprised when you conduct Baeckmo's simple stability test when the engine is re-installed - one thing in your favour is that your boat is relatively 'low' generally (no deck saloon or flying bridge) and she does not look top heavy.
Whereas that Northern Marine motor yacht does 'look' rather top heavy, from every aspect.

 

Oh thanks for posting that link to the Northern Marine - I should have done that. That boat does indeed look top heavy but looks can be deceiving. For instance look at a typical cruise ship. In the case of that particular Northern Marine trawler it indeed was extremely top heavy and the NTSB analysis shows it quantitatively. I believe that if ballasted as designed the Northern Marine would have been just fine and the NTSB report shows that also if I am understanding it correctly and that would be the top "23 LT Ballast No Heel Moment" curve. I believe that egregious manufacturing error put the company out of business.

Yes the sloshing would also reduce the stability and I hadn't even considered that in my list of conditions that would reduce it. It would be nice if fuel tanks had longitudinal baffles that would reduce that but I kind of doubt it does - maybe I'll try and take a peak inside. However as stated above, I'm liking the boat more with a reduced fuel load with everything else being nominal.

 

In my previous post I should have given a reference. The UN FAO has published a series of books on the themes of fishing and fishing vessels. In the "Fishing Boats of the World no 3" (ISBN 0 85238 043 7) there is a chapter written by P. Gurtner, with the title "Fishing boats for developing fisheries", where he discusses different stability criteria for safety and for crew comfort. I'll see if I can scan one of the diagrams that is showing the balance between too tender and too stiff.

 

I think I have something, somewhere relating to the FAO stability mentioned by Baeckmo above.

These two attachments below are also quite good.
The Stability Check illustrates well what Baeckmo was saying, re how the stability is ok if the roll period is less than the maximum beam.
If one allows for an increase in freeboard as the beam increases, then the relationship is approximately linear.

 

Thanks bajansailor! That check includes freeboard which I had not seen yet. Just an indication of stability of course around zero heel but it's easy to measure and I think it will be instructive. Also my freeboard is off scale high which is a good thing.

 

I suspect that your concern is the lighter engine causing the CG to be higher in the boat and more subject to lateral swaying.

I agree with the others in that the weight change is not all that significant. Probably it may move the CG more forward than upward. IIRC, the hull on the Formula 26 PC was 20 degree V. That may have more to do with stability than a weight reduction of the engine.

 

Here is the data on the 1989 Formula 26PC from their marketing brochure.

 

Thanks for your input tpenfield! Yes, I am a bit concerned about raising the the CG (and therefore reducing the GM) during my engine rebuild - actually it's more of an engine replacement not a rebuild. I believe that my when my fuel load is below around 1/2 the CG moves forward enough to make planing easier. That is the primary reason from swapping cast for aluminum and stainless, plus I'm doing the work myself and that cast stuff is heavy! Yes I do have that spec sheet for the boat and I also have some data from Thunderbird/Formula that indicates the dry weight is 8200 lbs not 7000 as indicated in their sales brochure which is quite a difference. I have "measured" a dry weight of 7500 lbs myself but that includes some estimates of trailer weight and remaining fuel so I have just bee using 8000 lbs as an estimate of dry weight.

My inclination of the deep-vee is that it just provides a lower place to put ballast (like fuel, batteries, engines, ...) and thus it provides a larger moment arm from the CG than say a flat bottom boat or a displacement hull. What is curious looking at the NTSB report of the Northern Marine accident it appears that the proper ballast was 23 LT, and they actually had 16 LT ballast. The total vessel weight was estimated to be 154 LT, and thus a ballast error of 7 LT (23-16) out of total 154 LT vessel caused the initial righting moment curve slope to be reduced to an unacceptable level. it also reduced the maximum righting moment by about 1/2, the point of instability heel angle from about 65 deg to 35 deg, and just eyeballing the total energy required to capsize by 1/3. Now to this error they also had an 8000 lb lateral ballast error off at some unspecified moment. This slid the entire moment arm curve downward about 0.15' and left a minuscule area of stability centered around 18 degrees. Multiple errors doomed that boat, but the first one is more problematic IMO!

Now 7 LT is 4.5% of 154 LT, and I am considering a change in total weight of around 3% so that is what is concerning me as I view both of these errors ballast type errors due to the engines low CG. I think my weight change can be considered ballast because the engine in the PC26 is down very close to the keel. Granted, there are almost no other similarities in the two vessels discussed however...

EDIT: The 8000 lb heeling moment ballast error looks to be 5.3 feet to port.

 

Sounds good. Do you have any Center of Gravity information on the PC 26 from Formula? As you probably know, there is a world of difference in the Northern Marine incident and your Formula PC26. I'd be more concerned about the metals you are using to lighten up the engine. IIRC, the PC26 had a single 454 ci engine (Mercruiser and the Bravo outdrive). The stock engine had cast iron dressing (Exhaust manifolds and risers). At one point the were stainless steel riser/elbows used, but many were replaced, because the stainless steel depleted the iron manifolds.

If you are thinking about a combination of iron, aluminum, and stainless steel, you probably want to consider if you are building a galvanic corrosion nightmare. Raw water cooling vs. closed cooling will also make a difference (but adds weight). Also, fresh water vs. salt water use is a concern.

FWIW - I have owned 2 Formulas a 1991 242SS and a 1996 330ss. The 330ss came with stainless steel riser/elbows on top of the regular cast iron exhaust manifolds. After 15 years in fresh water, the manifolds were quite depleted of metal, leaking , and needed replacement. I then switched the boat to a full closed cooling system, separating the coolant/water flow between the two pieces (manifold & riser) and never looked back.

Can you tell us more about the engine re-build and the various metals that get you down 300 lbs?

 

Thank you for that input tpenfield. Yes I converted the system to a closed cooling system, but it was too late in the life of the engine to save it. Originally the block, heads, intake manifold, exhaust manifold, and risers/elbows were cast iron and the exhaust collector and secondary "elbows" that connect the risers to the collector were aluminum. I now have purchased stainless exhaust manifolds and risers, aluminum heads, aluminum intake manifold and am currently deciding whether to go to an aluminum block. Yes its a big and expensive project compared to just buying a crate engine but I like this kind of stuff. I get a weight delta of 200 lbs with everything except the block, and 330 lbs if I go with an aluminum block and I added some weight for the heat exchanger. Raw water will exist only in the risers and beyond and of course in the heat exchanger.

Unfortunately I only have "approximate" data on the longitudinal CG of the PC26 directly from Formula, and nothing in the vertical. It looks like a nightmare to try and calculate the vertical CG. I know that the data exists somewhere at Formula but it's hard to get in contact with the people who would know where it is, plus being such an old boat doesn't help matters. Yes the two boats that I have been discussing could not be any different, but I just find it illuminating that the Northern Marine (Baaden) could capsize in the bay with what seems to be relatively small errors in ballast with respect to the total weight of the boat. Since I have been discussing that righting moment arm curve in that NTSB document I will post just that curve below with my annotations added which are simply based on my current understanding of these curves. Not having any experience in marine engineering, I welcome any corrections! I do have an engineering background however.