Another Repower Question

PeterPalmieri · Post by **PeterPalmieri** » Oct 3rd, '12, 11:21

So recent conventional wisdom in todays market says buy a diesel boat that's already done, or at least that's what I thought. Not that I was planning to go that route or repower soon but I think I had been convinced that it was the smarter money option. And the thought of repowering to diesel would be a losing proposition.

So let's run some quick math, a decent condition gas powered boat is going for about $30k. I wouldn't expect much more then that for my boat even with a few cosmetic upgrades. A professionally done cummins reman 270 repower is somewhere around 80k. That's a total cost of $110k, now I realize the listing prices on yacht world aren't selling prices but I don't see where the bargain in the already repowered boat is, unless your willing to take a chance on a boat with 2000+ hours and roll the dice that they've been maintained properly.

So what gives?

CaptPatrick · Post by **CaptPatrick** » Oct 3rd, '12, 11:50

So recent conventional wisdom in todays market says buy a diesel boat that's already done

That mainly applies to someone who doesn't already own a boat and wants diesel power. If you already own one and want the same boat with diesel, then repowering is a valid option...

As to asking/selling price, most people ask a price that have around a 25% drop in selling price factored in. Most boats asking, let's say $100K can be had for $75K. So in that case, a buying a $75k diesel boat could save $5k and selling an existing gas powered boat for $30K would bring the cost of the diesel boat down to $35k.

However, you have to figure what options you've added to the existing boat that don't come with the "new to you" boat, and that you will have/want to re-invest in...

There is no set formula. Every possibility is another set of number crunching. Until you're genuinely at the point of going one way or the other you're just spinning gears and going nowhere...

Br,

Patrick

mike ohlstein · Post by **mike ohlstein** » Oct 3rd, '12, 12:51

What value do you put on safety?

PeterPalmieri · Post by **PeterPalmieri** » Oct 3rd, '12, 13:09

Mike I really don't view a gas powered boat as unsafe, but my question really was related to buy a diesel or repower to a diesel, so in this case the end game is diesel.

Capt you are right on point except that I'm just spinning my wheels (which I often do). It's an important point of consideration as I plan to spend money on other upgrades on the boat.

captbone · Post by **captbone** » Oct 3rd, '12, 13:45

I think the other factor is that the boat you can buy for $110k will also have many other updated done such as interior, paint, helm pod, canvas......... already done. Your boat is very nice but after you did the diesel conversion you would still have to do all of that stuff to make it equal to what you can buy already done for basically the price of the repower and then still have your gas boat to sell.

Like stated before asking price is not selling price.

http://www.yachtworld.com/boats/1970/Be ... ted-States" onclick="window.open(this.href);return false;

CaptPatrick · Post by **CaptPatrick** » Oct 3rd, '12, 13:54

PeterPalmieri wrote:Capt you are right on point except that I'm just spinning my wheels (which I often do). It's an important point of consideration as I plan to spend money on other upgrades on the boat.

Peter,

Then you need to decide on buying a diesel boat before you continue modifications, or continue modifications and decide whether to stick with gas or repower to diesel.

If it were me, which it ain't, I'd repower to diesel, when and if the time came, and waste no more time crunching numbers for anything other than the boat I already had...

Bird in hand is worth two in bush.... Ah So, Grasshopper

Bruce · Post by **Bruce** » Oct 3rd, '12, 14:34

After 30 years and about 300 - 350 repowers of vessels ranging from sail boats to yachts, boats and justifying numbers don't mix.
You'll kick yourself silly trying to make the numbers work. You treat a boat like a fat old disgusting rich guy treats young arm candy. Which is you don't question what you spend because the enjoyment you get from the "happy ending" has no dollar value.

31 repowers are particularly evil. Only two out of the dozens I've done remained "basic" in nature. Everyone went beyond first wants and wishes but Tommy Drakes Island Girl because Tommy has tremendous self control and the 31 Double Mac which was just two brand new 6lp yannies hot off the design desk and a 3k install fee for a total of less than 40k.

Don't forget to include the "extras" because there is always extras.

SteveM · Post by **SteveM** » Oct 3rd, '12, 15:03

Peter, I tend to put everything in a spreadsheet.
When we finally decided to do a repower I also questioned selling and buying one already with diesels.
For us it didn't make sense because we REALLY like our boat how it is today. Aside from the engines it's what we want.
If you don't really like the boat and have lots of other stuff to do on it, and would prefer someone elses already done boat, then it may make sense.
But eventually I believe you move to diesels for safety, reliability, fuel consumption, longevity.
Keep your eyes peeled for a set of diesels and you may find a pair at a good price. You can do saved searches in Google and other search engines and it will email you whenever the words you specify show up on the web. Same thing with ebay.

Raybo Marine NY · Post by **Raybo Marine NY** » Oct 3rd, '12, 17:13

of course its cheaper to let someone else take the hit, but also keep in mind that when viewed in person most of these boats are not as appealing as they would seem in the ads. Maybe go and check a few out and see what you can REALLY get for your money.

PeterPalmieri · Post by **PeterPalmieri** » Oct 3rd, '12, 17:48

Thanks guys you are for the most part reenforcing that I'm going in the right direction.

Captbone as we all find preferences and takeaways from other projects its hard to find a boat with cosmetic upgrades that check all the boxes.

mike ohlstein · Post by **mike ohlstein** » Oct 4th, '12, 07:39

Peter,

I wasn't suggesting that diesel boats are safer than gas boats, per se. But that there are certain situations under which a diesel boat is much safer than a gas boat.

PeterPalmieri · Post by **PeterPalmieri** » Oct 4th, '12, 08:37

Please explain Mike. I'm not sure I follow.

mike ohlstein · Post by **mike ohlstein** » Oct 4th, '12, 10:22

Ever been 90 miles off shore with 10 five gallon gas cans and a beautiful weather forecast when it all turns to shit?

And now it's blowing 25 and the seas are rising, and you realize that you can't make it home without dumping that fuel into the tank? And even though you have a giant funnel, gasoline is blowing every which way........

I have, but never again. Diesel. Don't leave home without it.

Of course if you're never going off shore, it ain't no thang.

CaptPatrick · Post by **CaptPatrick** » Oct 4th, '12, 11:10

Add to that, according to the US Coast Guard, most fires and explosions on boats occur during or immediately after fueling.

Since diesel is pretty explosion resistant, I draw the assumption that the CG is almost exclusively referring to gasoline fires and explosions. I'm sure that a few are diesel, but very few.

A fill port with a faulty ground, high vapor, volatile liquid pulsing through the nozzle and a static electricity arc and you get my drift...

I've never had an uncomfortable moment at the diesel pump, but gasoline pumps and boats scare the hell out me...

Carl · Post by **Carl** » Oct 4th, '12, 11:39

CaptPatrick wrote: I've never had an uncomfortable moment at the diesel pump, but gasoline pumps and boats scare the hell out me...

I have had a very scary moment at the diesel pump. Boat topped off at a bit over a thousand gallons and owner went missing without leaving his credit card!!!

CaptPatrick · Post by **CaptPatrick** » Oct 4th, '12, 11:45

sim wrote:I have had a very scary moment at the diesel pump. Boat topped off at a bit over a thousand gallons and owner went missing without leaving his credit card!!!

Yup, that'd make me squirm too...

Craig Mac · Post by **Craig Mac** » Oct 4th, '12, 12:30

Besides the money, it really comes down to how are you going to use the boat? How many hours a season and how many trips will need that kind of range? Also, if you plan on doing any service yourself?

Although every year I promise myself to use the boat more, I am probably down to 50 hours a season. And my offshore trips are usually 30-60 miles, I feel very comfortable with 225 gallons of gas.

captbone · Post by **captbone** » Oct 4th, '12, 14:05

Craig Mac wrote:Besides the money, it really comes down to how are you going to use the boat? How many hours a season and how many trips will need that kind of range? Also, if you plan on doing any service yourself?

Although every year I promise myself to use the boat more, I am probably down to 50 hours a season. And my offshore trips are usually 30-60 miles, I feel very comfortable with 225 gallons of gas.

Well said. I came to this point when looking at new pickup trucks. I really wanted diesel but when I sat down and calculated my "real" use, it simply did not make sense. I had to be brutally honest because I really wanted diesel and found myself coming up with situations in my head to justify it but in reality it did not make sense for me. There is no argument that diesel is much nicer then gas inboards but I doubt many boaters really put the time/use to justify it over gas.

I guess that would be my last piece of advice, honestly and truly calculate your use/future predicted use before you go any further.

Brewster Minton · Post by **Brewster Minton** » Oct 4th, '12, 14:21

Just to stir the pot. Its a boat. It does not make sence. If its what you want, fine. Is it worth it? Yes if you are happy. Will you get your money back? Never. In the 17 years I have had mine it is one of the greatest things I have ever had. The time with my freinds and family, priceless. Have I spet more than I should have? Sure. But its what I want. Its not about being responsible. Pete make it the way you want. If it was the right thing you would get rid of the boats and put all that money tward your kids. I think your kids will remember going out on Bert with Dad more than how much it cost.

34Hatt · Post by **34Hatt** » Oct 4th, '12, 14:32

As for more on safety of diesels I have had to many gas engines not deliver when the throttle was used in a bad time. Inlets, running the locks when open and such always dreading the cough or stalling. My Diesels It never even a thought anymore, I untie 90% of the boat before I even start them. When warm and by my self I have totally untied the boat pushed off the bulkhead go to the bridge then start them and drive off never did that with any gas engine that I had owned so far.

captbone · Post by **captbone** » Oct 4th, '12, 14:41

Brewster Minton wrote:Just to stir the pot. Its a boat. It does not make sence. If its what you want, fine. Is it worth it? Yes if you are happy. Will you get your money back? Never. In the 17 years I have had mine it is one of the greatest things I have ever had. The time with my freinds and family, priceless. Have I spet more than I should have? Sure. But its what I want. Its not about being responsible. Pete make it the way you want. If it was the right thing you would get rid of the boats and put all that money tward your kids. I think your kids will remember going out on Bert with Dad more than how much it cost.

I cant help but stir the pot. ;)

I agree that the memories, time with family and enjoying the boat is very valuable. But he can do all of that as the boat sits and runs today. Logically there is no need to drop $80k into the boat unless he is using the boat 100+ hours per year or offshore every other trip. Having the best boat on the block is very nice but I dont know if having a fully funded retirement, 529 for kids, no debt, family vacations, 3 months reserve of safety money, no student loans, no mortgage,............. is better. I have none of these but must work toward some.

I know this is a boating website and boats dont make sense but throwing money into a boat (that runs) without regard to opportunity cost is something to be taken seriously. Just my 2 cents

Rocket · Post by **Rocket** » Oct 4th, '12, 17:26

I know that this is a somewhat heretical comment, but diesels aren't diesels any more. The new diesels with electronic controls are not like the old school mechanical ones that, if your fuel shut off failed, you had to stuff a pillow into the air intake to stop the damn thing. Fuel and fuelling is the major safety differnce as far as I can see. The new efi gassers are reliable, fuel efficient and capital cost effective. The 31 is a pretty heavy boat that goes fast for the size of gas motors available, but I think it unwise to immediatly crown diesel as king given the advancement in technology. In my industry diesel trucks are being replaced by CNG trucks for a myriad of reasons. Diesel is still dominant and all my trucks are diesels or diesel electric hybrids, but it is no where near the gulf in technology that it once was. If the emissions BS ever seriously hits the water the way it has hit the trucking industry prepare for unreliable expensive diesel engines.

bob lico · Post by **bob lico** » Oct 4th, '12, 17:35

Wow did I get a ear full today; my partner in the Maine house and former owner of S&K speed shop .now in retirement he persued bhis NCRA top judge standings for 63- 67 corvette stingray. He comes up with as a " matter of fact statement" number one killer of old corvette's is corn juice gas!!!!! Mike O ,,I had no idea today's gas is twice as volatile as the gas we used in our beloved sting rays back in the day.Couple of drops or vapor in tight area Bam up in flames. Listen to mike!!!!!! This is the family we are talking about.

In Memory Walter K · Post by **In Memory Walter K** » Oct 4th, '12, 18:34

Ask anyone who has converted if they're sorry they did. If you can afford to do it, do it. You won't be sorry. BUT, if you can't (nothing to be ashamed about), you still have a beautiful boat. Enjoy it!

mike ohlstein · Post by **mike ohlstein** » Oct 4th, '12, 19:33

Bob, Back in the day.....regular gas was 100 octane, and premium was 103 or 104. Sunoco had 106.

bob lico · Post by **bob lico** » Oct 4th, '12, 20:45

Most likely before my time . I was from Oceanside and Sunoco was 104 ,I ran it in my 1962 vette ,cam2 was 110 and you could go to republic airport and get the real deal -----green gas used for p-51 mustang and other high performance aircraft most often and sold on. 5 gallon cans 120 octane. For some reason this 10 percent ethanol used today ignites so easily in older cars because exhaust manifold is unprotected among other problems like ignition wires ,open coil . Sum it up bad situation for older big blocks in the confined of a Bertram engine compartment never design for ethanol .

Carl · Post by **Carl** » Oct 5th, '12, 06:20

Brewster Minton wrote:Just to stir the pot. Its a boat. It does not make sence. If its what you want, fine. Is it worth it? Yes if you are happy. Will you get your money back? Never. In the 17 years I have had mine it is one of the greatest things I have ever had. The time with my freinds and family, priceless. Have I spet more than I should have? Sure. But its what I want. Its not about being responsible. Pete make it the way you want. If it was the right thing you would get rid of the boats and put all that money tward your kids. I think your kids will remember going out on Bert with Dad more than how much it cost.

Stir the Pot??? Brewster you knocked the Pot over and danced in the mess.

Boats make total sense when you don't put boat in front of responsibilities. Boat can be a get-away, a way to be alone, a way to spend time with family and friends, a way to enjoy this world we live in. Putting all the money towards the kids is fine if that is what you feel is right...but when they grow up...okay when I look back at my time growing up...I remember more about being out on the water then most other memories...and its all good times. As I talk with others that never really spent time on the water...they missed something in life that money cannot give them. If you put a value on that as I do...boats make sense....

....now does it have to be a 31 Bertram with diesels, radar, GPS, AC, fresh Imron paint and Double Spreader Outriggers....I'm thinking yes, but I guess that point could be debatable.

For me...I won't put myself in hock for a boat. At least not while I have kids.

Pete...add up what you need to make your boat the way you want.
Then take that number and see what it will cost to buy one like that.

John F. · Post by **John F.** » Oct 5th, '12, 10:26

My 2 cents....its not even close.

The market for used boats has crashed. Its pretty amazing to me what you can get compared to what you could have gotten 10 years ago for the same $. There are some really nicely redone B31s out there that can be had for some fraction of what they cost to build--take the resale of Buddy Boy for example. Aside from the repower, alot of these boats have had the ac side of the electrical redone, a new 1/2 tower and pipework, new outriggers, and new paint. Add that up.

The other problem with gas boats is that you can't buy new big-blocks anymore. Everything is a rebuild or reman. Some guys have had good luck with remans, others not so much. If you go new, with say a merc. or crusader 6.0 or 6.1 small block, you'd be OK. But, you'd lose the best thing about the 454s---they're simple, with cheap parts, and you can fix anything on them for cheap. So, if one of your motors lets go, then you have to make a big decision--rebuild the BBs one at a time and stick with them, go for new sbs, or go diesel.

Gassers are cheap to own and fix, so they make sense if your intended use isn't running to the canyons (range). But, I found even running around the Bay, we'd take the B20 alot rather than the B31. The B31 was great, but got about 1 nmpg. The B20 is great, and gets about 3 nmpg. My kids were in HS, so the weekends on the boat--we had so much fun--were a thing of the past. So, my B31 sat, and with kids going to college, I put an ad on THT just before the market went bust. She didn't sell...at first.

Out of the blue, a guy called that saw my 2 year old or so ad on THT that I hadn't updated in a long while, and bought my boat. I almost didn't sell her, but with a kid in college and one about to go (they're both in college now), the money was nice to have (sadly, basically 1 year tuition). She's in Florida, at American Custom Yachts, and she's going to get topside paint. The owner--nice guy--calls me every few months for an update. I like that.

So basically, there is no way in this economy, with the boat market as it is, to make the numbers work on a repower/restoration. It made perfect economical sense for me to sell my B31, and plan to get a diesel B31 some time in the future that's been redone or partially redone, and let the seller take the hit.

However, I can tell you, that if I had to do it over again, I wouldn't sell my B31. I miss her. I had a great time with my family and friends on that boat, and now and then, fished her alone. She was/is a great boat. Boats make no economic sense, but if you love 'em the way the faithful here do, B31s, all of them, make perfect sense.

So Pete, I've been partially down the path you're thinking about. From a purely money point of view, use your B31 as is, make sure she's safe, and sell her and get a diesel boat that's been redone when you can afford it. Although I had put alot of time and money into my B31, and had a ways to go, that's what I decided to do and sold my Anna E.

And having made that economically sound decision, I can tell you with all sincerity, I screwed up. Totally screwed up. Brewster hit it on the head.

Again, my 2 cents.

CaptPatrick · Post by **CaptPatrick** » Oct 5th, '12, 11:31

John F wrote:From a purely money point of view, use your B31 as is, make sure she's safe, and sell her and get a diesel boat that's been redone when you can afford it.

That will hold true as long as the market stays depressed. When, and if, the market comes back those days of "steal a boat" will be gone...

Hueso · Post by **Hueso** » Oct 5th, '12, 13:22

John:

When you were about to put some sense in my head.......I'm back to square one.............REPOWER!.....REPOWER!.......LMAO!

PeterPalmieri · Post by **PeterPalmieri** » Oct 5th, '12, 14:23

Some really thoughtful comments. At the moment I can afford smaller projects like top sides Raybo had redid over the winter. I really couldn't afford right now to buy a diesel boat or lay out the money for a proper repower. I've been pretty smart with my boating and fishing addiction and pay outright no borrowing money. I guess when I am forced to repower or have the cash to do so it would be a better time to think on this type of decision.

I do like the idea of the boat being eventually 100% redone to my liking and can justify that over a boat that was done for someone else, even if it costs more.

We got a babysitter this morning to watch the 2 month old, took Shannon and carter out for a few hours. The smiles are priceless. If the boat was frozen in its current state for many years we'd still be happy but there is a dream of having the boat perfect and every small upgrade brings me a step closer to the dream.

Another note, I'd rather not start another thread replaced all the guages with VDOs and I can't get the tachs synced up. Mechanic and electrician both called VDO. It's set at 8 pulses I believe and the tachs show a much higher RPM then what the motors are at. Idle is about 1000, what should be my cruising speed at 19 knots is well over 3000 and WOT pins the tachs at over 6000. Setting it to 4 pulses the tach reads double what it should. Mechanic checked RPM at the motor and we are all normal. Customer service wasn't much help said it should be set at 8. Any ideas?

John F. · Post by **John F.** » Oct 5th, '12, 14:45

Peter-

Outside of the fact that you can have your B31 just how you want, I was able to pay as I went on the upgrades.

John

PeterPalmieri · Post by **PeterPalmieri** » Oct 5th, '12, 15:13

John, well said the pay as you go method is what works financially for me. BTW I also have a 20' skiff that's my fishing boat that is no family boat just fishing.

Tony Meola · Post by **Tony Meola** » Oct 5th, '12, 22:23

Peter

You will never repower on the cheap. There are too many while I am here I might as well do it that comes up. I bet my engines and Trannies were less than half the cost of the total repower. There was replacing the bulkhead behind the engines, redo the strut pads etc. Yes you might find a diesel for 70 but make sure all that stuff is done, otherwise you are back at square one.

You know what, I am so happy I dropped in the 270 remans. Totally different boat. Yes I could have bought one cheaper, but you know what too much in sentimental value. My dad bought it new in 1975. I know every inch and of that boat. When my dad died I kept her. When he was alive, he kept the engines running and I took care of everything else. Is she totally redone no. But she is clean and functional and still turns heads.

What more could you want.

Tony Meola · Post by **Tony Meola** » Oct 5th, '12, 22:26

mike ohlstein wrote:Bob, Back in the day.....regular gas was 100 octane, and premium was 103 or 104. Sunoco had 106.

Mike

My father sold Sunoco gas for years. At first you could not dial more than the 240 option. The 260 option was blocked out because it contained too much lead and was way more than 106 Octane. Then they dropped the lead count, and you could dial in the highest octane available at the pump, that was 260 at 106 Octane. Some time in the 80's they dropped it to 104 Octane.

mike ohlstein · Post by **mike ohlstein** » Oct 6th, '12, 07:24

In 1969 I bought a frame out of the Heathkit catalogue and pulled a 2.5 HP Briggs and Stratton engine out of a junked lawn mower and I was in business. For about thirty cents worth of 100 octane, I could ride for hours.

Charlie J · Post by **Charlie J** » Oct 6th, '12, 17:22

didnt sunnoco have the 260

Tony Meola · Post by **Tony Meola** » Oct 6th, '12, 23:04

Charlie J wrote:didnt sunnoco have the 260

Charlie yes they did. At the time it was the highest Octane you could buy. There was a time when they locked it out and you could not pump it because it had too much lead in it. Then they took some of the lead out and called it 260. Good old blue sunoco.

Keith Poe · Post by **Keith Poe** » Oct 7th, '12, 04:17

I bought this 31-b for 20-K with 500 hours on the motors pretty good deal.

I bought this 7.4 Mercruiser MPI with 200 hours on it from the great lakes area never been salted for my 24' skip jack for $3500.00 on E bay.

http://youtu.be/ZqbEfvI6i3E" onclick="window.open(this.href);return false;

.

Keith Poe · Post by **Keith Poe** » Oct 7th, '12, 20:04

Was thinking going down the road this afternoon it would be nice in a perfect world to get a loan against your boat then buy one with Diesels then when and if the market increases sell the old one.

CaptPatrick · Post by **CaptPatrick** » Oct 7th, '12, 20:29

A loan on a 50 year old boat? In todays banking market? That's not a perfect world, that's a fantasy world...

Carl · Post by **Carl** » Oct 8th, '12, 09:17

Want a loan against your boat??? How much fuel ya got in the tank?

PeterPalmieri · Post by **PeterPalmieri** » Oct 9th, '12, 08:27

So I replaced my old guages with VDO viewline. As it turns out the tachs I bought will not calibrate with my 1985 454s, the highest setting on the tachs is 8 impulses. So I'm having new tachs sent to me with the digital hour meter this tach can be set from .5 - 399 impulses. When I swap them out will I be able to calibrate without taking RPM readings directly from the engine?

I am hoping to make it easy, if I have to run the boat back to the mechanic to have him help me calibrate it's not the biggest deal...

Carl · Post by **Carl** » Oct 9th, '12, 09:05

Pete,
I'm real good with electronics.
If you have a problem...just bang on them frantically while citing expletives.
Works like a charm...even if they still do not work, at least now you know why.
Carl

CamB25 · Post by **CamB25** » Oct 9th, '12, 09:49

Looks like the dip switches can be set to one of eight choices for impulses per revolution. The chart:

http://www.vdo.com/generator/www/com/en ... rect=false" onclick="window.open(this.href);return false;

It depends on where you pick up the tach siganl on your engines. If memory serves me correctly, on old school "analog" engines, the tach is driven from the coil. In this case the Impulses per revolution for a V-8 is 8.

PeterPalmieri · Post by **PeterPalmieri** » Oct 9th, '12, 10:13

Cam those are the tachs that didn't work, set to 8 the guages read high and that is the max setting. Replacing those with the tach that has a small digital display and can be set higher

PeterPalmieri · Post by **PeterPalmieri** » Oct 9th, '12, 10:25

http://marine.vdo-gauges.com/instrument ... 2-24v.html" onclick="window.open(this.href);return false;

CamB25 · Post by **CamB25** » Oct 9th, '12, 21:06

wellllll....I can't imagine the new tach is any different than the old one. A tach is a simple pulse counter, it's looking for a leading or trailing edge of the ignition signal that drives the high voltage to the plugs. There should be no calibration necessary. Do you have electronic ignition or points? If EI, double check the wiring to the module to ensure it's in the right location. Check wiriing - a dirty signal will make tach erratic. What was wrong with the old tachs?

Bruce · Post by **Bruce** » Oct 10th, '12, 07:38

CamB25 wrote:wellllll....I can't imagine the new tach is any different than the old one. A tach is a simple pulse counter, it's looking for a leading or trailing edge of the ignition signal that drives the high voltage to the plugs. There should be no calibration necessary. Do you have electronic ignition or points? If EI, double check the wiring to the module to ensure it's in the right location. Check wiriing - a dirty signal will make tach erratic. What was wrong with the old tachs?

The reason for the adjustment is to allow the manufacturer to use one tach for various engine models to a point.
Electronic ignition uses 3 different dist pickups, variable reluctor sensor, a hall sensor, or an optical sensor.
There are some differences in the 3 signal waveforms.
Tachs can also be used between 2 and 4 cycle with adjustments.

Diesel can use altenator(a.c. tap), tach generator drive off a cable or cam shaft or a flywheel pulse pickup. Signals are different.